diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 22d67a30f1..9f31426e61 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -308,6 +308,8 @@ module ApplicationHelper
 
     css << "ee-banners-#{EnterpriseToken.show_banners? ? 'visible' : 'hidden'}"
 
+    css << "env-#{Rails.env}"
+
     # Add browser specific classes to aid css fixes
     css += browser_specific_classes
 
diff --git a/docker-compose.yml b/docker-compose.yml
index 9204582884..5200b4dd02 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -167,6 +167,7 @@ services:
       - "opdata:/var/openproject/assets"
       - "bundle:/usr/local/bundle"
       - "tmp-test:/home/dev/openproject/tmp"
+      - "./tmp/capybara:/home/dev/openproject/tmp/capybara"
 
   # https://vitobotta.com/2019/09/04/rails-parallel-system-tests-selenium-docker/
   selenium-hub:
diff --git a/frontend/src/app/core/state/collection-store.ts b/frontend/src/app/core/state/collection-store.ts
index 5f3f351a4b..cbc08aa4bc 100644
--- a/frontend/src/app/core/state/collection-store.ts
+++ b/frontend/src/app/core/state/collection-store.ts
@@ -93,7 +93,8 @@ export function selectEntitiesFromIDCollection<T extends CollectionItem>(service
  * @param state
  * @param params
  */
-export function selectCollectionAsEntities$<T extends CollectionItem>(service:CollectionService<T>, state:CollectionState<T>, params:Apiv3ListParameters):T[] {
+export function selectCollectionAsEntities$<T extends CollectionItem>(service:CollectionService<T>, params:Apiv3ListParameters):T[] {
+  const state = service.query.getValue();
   const key = collectionKey(params);
   const collection = state.collections[key];
 
diff --git a/frontend/src/app/core/state/openproject-state.module.ts b/frontend/src/app/core/state/openproject-state.module.ts
index b508ea908c..391137755b 100644
--- a/frontend/src/app/core/state/openproject-state.module.ts
+++ b/frontend/src/app/core/state/openproject-state.module.ts
@@ -31,11 +31,13 @@ import {
 } from '@angular/core';
 import { InAppNotificationsResourceService } from './in-app-notifications/in-app-notifications.service';
 import { ProjectsResourceService } from './projects/projects.service';
+import { PrincipalsResourceService } from './principals/principals.service';
 
 @NgModule({
   providers: [
     InAppNotificationsResourceService,
     ProjectsResourceService,
+    PrincipalsResourceService,
   ],
 })
 export class OpenProjectStateModule {
diff --git a/frontend/src/app/core/state/principals/group.model.ts b/frontend/src/app/core/state/principals/group.model.ts
new file mode 100644
index 0000000000..9fd2628ff2
--- /dev/null
+++ b/frontend/src/app/core/state/principals/group.model.ts
@@ -0,0 +1,13 @@
+import { ID } from '@datorama/akita';
+import { HalResourceLinks } from 'core-app/core/state/hal-resource';
+
+export interface GroupHalResourceLinks extends HalResourceLinks { }
+
+export interface Group {
+  id:ID;
+  name:string;
+  createdAt:string;
+  updatedAt:string;
+
+  _links:GroupHalResourceLinks;
+}
diff --git a/frontend/src/app/core/state/principals/placeholder-user.model.ts b/frontend/src/app/core/state/principals/placeholder-user.model.ts
new file mode 100644
index 0000000000..3dc91212b6
--- /dev/null
+++ b/frontend/src/app/core/state/principals/placeholder-user.model.ts
@@ -0,0 +1,17 @@
+import { ID } from '@datorama/akita';
+import { HalResourceLink, HalResourceLinks } from 'core-app/core/state/hal-resource';
+
+export interface PlaceholderUserHalResourceLinks extends HalResourceLinks {
+  updateImmediately:HalResourceLink;
+  delete:HalResourceLink;
+  showUser:HalResourceLink;
+}
+
+export interface PlaceholderUser {
+  id:ID;
+  name:string;
+  createdAt:string;
+  updatedAt:string;
+
+  _links:PlaceholderUserHalResourceLinks;
+}
diff --git a/frontend/src/app/core/state/principals/principal.model.ts b/frontend/src/app/core/state/principals/principal.model.ts
new file mode 100644
index 0000000000..41baf6dcd6
--- /dev/null
+++ b/frontend/src/app/core/state/principals/principal.model.ts
@@ -0,0 +1,5 @@
+import { User } from './user.model';
+import { Group } from './group.model';
+import { PlaceholderUser } from './placeholder-user.model';
+
+export type Principal = User|Group|PlaceholderUser;
diff --git a/frontend/src/app/core/state/principals/principals.query.ts b/frontend/src/app/core/state/principals/principals.query.ts
new file mode 100644
index 0000000000..ec5d89563e
--- /dev/null
+++ b/frontend/src/app/core/state/principals/principals.query.ts
@@ -0,0 +1,10 @@
+import { QueryEntity } from '@datorama/akita';
+import { Observable } from 'rxjs';
+import { Principal } from './principal.model';
+import { PrincipalsState } from './principals.store';
+
+export class PrincipalsQuery extends QueryEntity<PrincipalsState> {
+  public byIds(ids:string[]):Observable<Principal[]> {
+    return this.selectMany(ids);
+  }
+}
diff --git a/frontend/src/app/core/state/principals/principals.service.ts b/frontend/src/app/core/state/principals/principals.service.ts
new file mode 100644
index 0000000000..59603bdb7e
--- /dev/null
+++ b/frontend/src/app/core/state/principals/principals.service.ts
@@ -0,0 +1,110 @@
+import { Injectable } from '@angular/core';
+import {
+  catchError,
+  tap,
+} from 'rxjs/operators';
+import { Observable } from 'rxjs';
+import {
+  applyTransaction,
+  ID,
+} from '@datorama/akita';
+import { APIV3Service } from 'core-app/core/apiv3/api-v3.service';
+import { ToastService } from 'core-app/shared/components/toaster/toast.service';
+import { IHALCollection } from 'core-app/core/apiv3/types/hal-collection.type';
+import { HttpClient } from '@angular/common/http';
+import { PrincipalsQuery } from 'core-app/core/state/principals/principals.query';
+import { Apiv3ListParameters } from 'core-app/core/apiv3/paths/apiv3-list-resource.interface';
+import { collectionKey } from 'core-app/core/state/collection-store';
+import {
+  EffectHandler,
+} from 'core-app/core/state/effects/effect-handler.decorator';
+import { ActionsService } from 'core-app/core/state/actions/actions.service';
+import { PrincipalsStore } from './principals.store';
+import { Principal } from './principal.model';
+
+@EffectHandler
+@Injectable()
+export class PrincipalsResourceService {
+  protected store = new PrincipalsStore();
+
+  readonly query = new PrincipalsQuery(this.store);
+
+  private get principalsPath():string {
+    return this
+      .apiV3Service
+      .principals
+      .path;
+  }
+
+  constructor(
+    readonly actions$:ActionsService,
+    private http:HttpClient,
+    private apiV3Service:APIV3Service,
+    private toastService:ToastService,
+  ) {
+  }
+
+  fetchPrincipals(params:Apiv3ListParameters):Observable<IHALCollection<Principal>> {
+    const collectionURL = collectionKey(params);
+
+    return this
+      .http
+      .get<IHALCollection<Principal>>(this.principalsPath + collectionURL)
+      .pipe(
+        tap((events) => {
+          applyTransaction(() => {
+            this.store.add(events._embedded.elements);
+            this.store.update(({ collections }) => (
+              {
+                collections: {
+                  ...collections,
+                  [collectionURL]: {
+                    ...collections[collectionURL],
+                    ids: events._embedded.elements.map((el) => el.id),
+                  },
+                },
+              }
+            ));
+          });
+        }),
+        catchError((error) => {
+          this.toastService.addError(error);
+          throw error;
+        }),
+      );
+  }
+
+  update(id:ID, principal:Partial<Principal>):void {
+    this.store.update(id, principal);
+  }
+
+  modifyCollection(params:Apiv3ListParameters, callback:(collection:ID[]) => ID[]):void {
+    const key = collectionKey(params);
+    this.store.update(({ collections }) => (
+      {
+        collections: {
+          ...collections,
+          [key]: {
+            ...collections[key],
+            ids: [...callback(collections[key]?.ids || [])],
+          },
+        },
+      }
+    ));
+  }
+
+  removeFromCollection(params:Apiv3ListParameters, ids:ID[]):void {
+    const key = collectionKey(params);
+    this.store.update(({ collections }) => (
+      {
+        collections: {
+          ...collections,
+          [key]: {
+            ...collections[key],
+            ids: (collections[key]?.ids || []).filter((id) => !ids.includes(id)),
+          },
+        },
+      }
+    ));
+  }
+}
diff --git a/frontend/src/app/core/state/principals/principals.store.ts b/frontend/src/app/core/state/principals/principals.store.ts
new file mode 100644
index 0000000000..19b8fae8b2
--- /dev/null
+++ b/frontend/src/app/core/state/principals/principals.store.ts
@@ -0,0 +1,13 @@
+import { EntityStore, StoreConfig } from '@datorama/akita';
+import { Principal } from './principal.model';
+import { CollectionState, createInitialCollectionState } from 'core-app/core/state/collection-store';
+
+export interface PrincipalsState extends CollectionState<Principal> {
+}
+
+@StoreConfig({ name: 'principals' })
+export class PrincipalsStore extends EntityStore<PrincipalsState> {
+  constructor() {
+    super(createInitialCollectionState());
+  }
+}
diff --git a/frontend/src/app/core/state/principals/user.model.ts b/frontend/src/app/core/state/principals/user.model.ts
new file mode 100644
index 0000000000..60f6282d61
--- /dev/null
+++ b/frontend/src/app/core/state/principals/user.model.ts
@@ -0,0 +1,31 @@
+import { ID } from '@datorama/akita';
+import { HalResourceLink, HalResourceLinks } from 'core-app/core/state/hal-resource';
+
+export interface UserHalResourceLinks extends HalResourceLinks {
+  lock:HalResourceLink;
+  unlock:HalResourceLink;
+  delete:HalResourceLink;
+  showUser:HalResourceLink;
+}
+
+export interface User {
+  id:ID;
+  name:string;
+  createdAt:string;
+  updatedAt:string;
+
+  // Properties
+  login:string;
+
+  firstName:string;
+
+  lastName:string;
+
+  email:string;
+
+  avatar:string;
+
+  status:string;
+
+  _links:UserHalResourceLinks;
+}
diff --git a/frontend/src/app/features/in-app-notifications/center/state/ian-center.service.ts b/frontend/src/app/features/in-app-notifications/center/state/ian-center.service.ts
index c40417b191..37a3563ba3 100644
--- a/frontend/src/app/features/in-app-notifications/center/state/ian-center.service.ts
+++ b/frontend/src/app/features/in-app-notifications/center/state/ian-center.service.ts
@@ -137,7 +137,7 @@ export class IanCenterService extends UntilDestroyedMixin {
   }
 
   markAllAsRead():void {
-    const ids:ID[] = selectCollectionAsEntities$(this.resourceService, this.resourceService.query.getValue(), this.query.params)
+    const ids:ID[] = selectCollectionAsEntities$(this.resourceService, this.query.params)
       .filter((notification) => notification.readIAN === false)
       .map((notification) => notification.id);
 
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.html b/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.html
new file mode 100644
index 0000000000..f87631480d
--- /dev/null
+++ b/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.html
@@ -0,0 +1,8 @@
+<op-autocompleter
+  data-qa-selector="tp-add-assignee"
+  (change)="selectUser($event)"
+  [fetchDataDirectly]="true"
+  [getOptionsFn]="getOptionsFn"
+  resource="user"
+  appendTo="body"
+></op-autocompleter>
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.ts b/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.ts
new file mode 100644
index 0000000000..2329aff784
--- /dev/null
+++ b/frontend/src/app/features/team-planner/team-planner/planner/add-assignee.component.ts
@@ -0,0 +1,96 @@
+// -- copyright
+// OpenProject is an open source project management software.
+// Copyright (C) 2012-2021 the OpenProject GmbH
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public License version 3.
+//
+// OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+// Copyright (C) 2006-2013 Jean-Philippe Lang
+// Copyright (C) 2010-2013 the ChiliProject Team
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public License
+// as published by the Free Software Foundation; either version 2
+// of the License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+//
+// See COPYRIGHT and LICENSE files for more details.
+//++
+
+import {
+  Component,
+  ElementRef,
+  EventEmitter,
+  Injector,
+  Input,
+  Output,
+  ChangeDetectionStrategy,
+} from '@angular/core';
+import { HalResourceService } from 'core-app/features/hal/services/hal-resource.service';
+import { PathHelperService } from 'core-app/core/path-helper/path-helper.service';
+import { I18nService } from 'core-app/core/i18n/i18n.service';
+import { Observable } from 'rxjs';
+import { map } from 'rxjs/operators';
+import { CurrentProjectService } from 'core-app/core/current-project/current-project.service';
+import { HalResourceNotificationService } from 'core-app/features/hal/services/hal-resource-notification.service';
+import { HalResource } from 'core-app/features/hal/resources/hal-resource';
+import { ApiV3FilterBuilder } from 'core-app/shared/helpers/api-v3/api-v3-filter-builder';
+import { APIV3Service } from 'core-app/core/apiv3/api-v3.service';
+
+@Component({
+  templateUrl: './add-assignee.component.html',
+  selector: 'op-tp-add-assignee',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+})
+export class AddAssigneeComponent {
+  @Output() public selectAssignee = new EventEmitter<HalResource>();
+
+  @Input() alreadySelected:string[] = [];
+
+  public getOptionsFn = (query:string):Observable<unknown[]> => this.autocomplete(query);
+
+  constructor(
+    protected elementRef:ElementRef,
+    protected halResourceService:HalResourceService,
+    protected I18n:I18nService,
+    protected halNotification:HalResourceNotificationService,
+    readonly pathHelper:PathHelperService,
+    readonly apiV3Service:APIV3Service,
+    readonly injector:Injector,
+    readonly currentProjectService:CurrentProjectService,
+  ) { }
+
+  public autocomplete(term:string|null):Observable<HalResource[]> {
+    const filters = new ApiV3FilterBuilder();
+
+    filters.add('member', '=', [this.currentProjectService.id || '']);
+
+    if (term) {
+      filters.add('name_and_identifier', '~', [term]);
+    }
+
+    return this
+      .apiV3Service
+      .principals
+      .filtered(filters)
+      .get()
+      .pipe(
+        map((collection) => collection.elements.filter(
+          (user) => !this.alreadySelected.find((selected) => selected === user.id),
+        )),
+      );
+  }
+
+  public selectUser(user:HalResource):void {
+    this.selectAssignee.emit(user);
+  }
+}
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.html b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.html
index 0d9fd43010..d66b1708b2 100644
--- a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.html
+++ b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.html
@@ -11,8 +11,45 @@
     [textContent]="calendar.tooManyResultsText"
     class="op-wp-calendar--notification"></div>
 </ng-container>
+
 <ng-template #resourceContent let-resource="resource">
-  <op-principal
-    [principal]="resource.extendedProps.user"
-  ></op-principal>
+  <div
+    *ngIf="resource.extendedProps.principal"
+    class="tp-assignee"
+  >
+    <op-principal
+      [principal]="resource.extendedProps.principal"
+      class="tp-assignee--principal"
+    ></op-principal>
+    <button
+      type="button"
+      class="tp-assignee--remove"
+      (click)="removeAssignee(resource.id)"
+      [aria-label]="text.remove_assignee"
+      [attr.data-qa-remove-assignee]="resource.extendedProps.principal.id"
+    >
+      <op-icon icon-classes="icon-remove"></op-icon>
+    </button>
+  </div>
+
+  <op-tp-add-assignee
+    *ngIf="!resource.extendedProps.principal"
+    (selectAssignee)="addAssignee($event)"
+    [alreadySelected]="principalIds$ | async"
+  ></op-tp-add-assignee>
 </ng-template>
+
+<div
+  class="wp-inline-create-button"
+  *ngIf="!(showAddAssignee$ | async)"
+>
+  <button
+    type="button"
+    class="wp-inline-create--add-link tp-assignee-add-button"
+    (click)="showAssigneeAddRow()"
+    data-qa-selector="tp-assignee-add-button"
+  >
+    <op-icon icon-classes="icon-context icon-add"></op-icon>
+    <span [textContent]="text.add_assignee"></span>
+  </button>
+</div>
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.sass b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.sass
index 0bebb94376..302ab38e75 100644
--- a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.sass
+++ b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.sass
@@ -6,3 +6,6 @@
   overflow: auto
   // Avoid empty space on the right since the styled scrollbar would only be visible on hover.
   @include no-visible-scroll-bar
+
+.tp-assignee-add-button
+  margin-top: 1rem
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.ts b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.ts
index c348ff2f3a..b05c4e3be9 100644
--- a/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.ts
+++ b/frontend/src/app/features/team-planner/team-planner/planner/team-planner.component.ts
@@ -11,27 +11,34 @@ import {
   CalendarOptions,
   EventInput,
 } from '@fullcalendar/core';
+import {
+  combineLatest,
+  Subject,
+} from 'rxjs';
+import {
+  debounceTime,
+  mergeMap,
+  map,
+  filter,
+  distinctUntilChanged,
+} from 'rxjs/operators';
+import { EventClickArg } from '@fullcalendar/common';
+import { StateService } from '@uirouter/angular';
 import resourceTimelinePlugin from '@fullcalendar/resource-timeline';
+import { FullCalendarComponent } from '@fullcalendar/angular';
 import { I18nService } from 'core-app/core/i18n/i18n.service';
 import { ConfigurationService } from 'core-app/core/config/configuration.service';
-import { FullCalendarComponent } from '@fullcalendar/angular';
 import { EventViewLookupService } from 'core-app/features/team-planner/team-planner/planner/event-view-lookup.service';
-import { States } from 'core-app/core/states/states.service';
-import { StateService } from '@uirouter/angular';
-import { DomSanitizer } from '@angular/platform-browser';
 import { WorkPackageViewFiltersService } from 'core-app/features/work-packages/routing/wp-view-base/view-services/wp-view-filters.service';
-import { WorkPackagesListService } from 'core-app/features/work-packages/components/wp-list/wp-list.service';
 import { IsolatedQuerySpace } from 'core-app/features/work-packages/directives/query-space/isolated-query-space';
-import { WorkPackagesListChecksumService } from 'core-app/features/work-packages/components/wp-list/wp-list-checksum.service';
-import { SchemaCacheService } from 'core-app/core/schemas/schema-cache.service';
 import { CurrentProjectService } from 'core-app/core/current-project/current-project.service';
-import { OpTitleService } from 'core-app/core/html/op-title.service';
-import { Subject } from 'rxjs';
+import { splitViewRoute } from 'core-app/features/work-packages/routing/split-view-routes.helper';
+import { QueryFilterInstanceResource } from 'core-app/features/hal/resources/query-filter-instance-resource';
+import { PrincipalsResourceService } from 'core-app/core/state/principals/principals.service';
+import { Apiv3ListParameters } from 'core-app/core/apiv3/paths/apiv3-list-resource.interface';
 import { WorkPackageResource } from 'core-app/features/hal/resources/work-package-resource';
 import { HalResource } from 'core-app/features/hal/resources/hal-resource';
 import { UntilDestroyedMixin } from 'core-app/shared/helpers/angular/until-destroyed.mixin';
-import { debounceTime } from 'rxjs/operators';
-import { APIV3Service } from 'core-app/core/apiv3/api-v3.service';
 import { ResourceLabelContentArg } from '@fullcalendar/resource-common';
 import { OpCalendarService } from 'core-app/shared/components/calendar/op-calendar.service';
 import { WorkPackageCollectionResource } from 'core-app/features/hal/resources/wp-collection-resource';
@@ -56,28 +63,58 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
 
   @ViewChild('resourceContent') resourceContent:TemplateRef<unknown>;
 
+  @ViewChild('assigneeAutocompleter') assigneeAutocompleter:TemplateRef<unknown>;
+
+  private resizeSubject = new Subject<unknown>();
+
   calendarOptions$ = new Subject<CalendarOptions>();
 
   projectIdentifier:string|undefined = undefined;
 
+  showAddAssignee$ = new Subject<boolean>();
+
+  private principalIds$ = this.wpTableFilters
+    .live$()
+    .pipe(
+      this.untilDestroyed(),
+      map((queryFilters) => {
+        const assigneeFilter = queryFilters.find((queryFilter) => queryFilter._type === 'AssigneeQueryFilter');
+        return ((assigneeFilter?.values || []) as HalResource[]).map((p) => p.id);
+      }),
+    );
+
+  private params$ = this.principalIds$
+    .pipe(
+      this.untilDestroyed(),
+      filter((ids) => ids.length > 0),
+      map((ids) => ({
+        filters: [['id', '=', ids]],
+      }) as Apiv3ListParameters),
+    );
+
+  assignees:HalResource[] = [];
+
   text = {
     assignees: this.I18n.t('js.team_planner.label_assignee_plural'),
+    add_assignee: this.I18n.t('js.team_planner.add_assignee'),
+    remove_assignee: this.I18n.t('js.team_planner.remove_assignee'),
   };
 
+  principals$ = this.principalIds$
+    .pipe(
+      this.untilDestroyed(),
+      mergeMap((ids:string[]) => this.principalsResourceService.query.byIds(ids)),
+      debounceTime(50),
+      distinctUntilChanged((prev, curr) => prev.length === curr.length && prev.length === 0),
+    );
+
   constructor(
-    private elementRef:ElementRef,
-    private states:States,
     private $state:StateService,
-    private sanitizer:DomSanitizer,
     private configuration:ConfigurationService,
-    private apiV3Service:APIV3Service,
+    private principalsResourceService:PrincipalsResourceService,
     private wpTableFilters:WorkPackageViewFiltersService,
-    private wpListService:WorkPackagesListService,
     private querySpace:IsolatedQuerySpace,
-    private wpListChecksumService:WorkPackagesListChecksumService,
-    private schemaCache:SchemaCacheService,
     private currentProject:CurrentProjectService,
-    private titleService:OpTitleService,
     private viewLookup:EventViewLookupService,
     private I18n:I18nService,
     readonly calendar:OpCalendarService,
@@ -86,18 +123,64 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
   }
 
   ngOnInit():void {
-    this.setupWorkPackagesListener();
     this.initializeCalendar();
     this.projectIdentifier = this.currentProject.identifier ? this.currentProject.identifier : undefined;
 
-    this.calendar.resize$
+    this
+      .querySpace
+      .results
+      .values$()
+      .pipe(this.untilDestroyed())
+      .subscribe(() => {
+        this.ucCalendar.getApi().refetchEvents();
+      });
+
+    this.resizeSubject
+      .pipe(this.untilDestroyed())
+      .subscribe(() => {
+        this.ucCalendar.getApi().updateSize();
+      });
+
+    this.params$
+      .pipe(this.untilDestroyed())
+      .subscribe((params) => {
+        this.principalsResourceService.fetchPrincipals(params).subscribe();
+      });
+
+    combineLatest([
+      this.principals$,
+      this.showAddAssignee$,
+    ])
       .pipe(
         this.untilDestroyed(),
-        debounceTime(50),
+        debounceTime(0),
       )
-      .subscribe(() => {
-        this.ucCalendar.getApi().updateSize();
+      .subscribe(([principals, showAddAssignee]) => {
+        const api = this.ucCalendar.getApi();
+
+        api.getResources().forEach((resource) => resource.remove());
+
+        principals.forEach((principal) => {
+          const { self } = principal._links;
+          const id = Array.isArray(self) ? self[0].href : self.href;
+          api.addResource({
+            principal,
+            id,
+            title: principal.name,
+          });
+        });
+
+        if (showAddAssignee) {
+          api.addResource({
+            principal: null,
+            id: 'NEW',
+            title: '',
+          });
+        }
       });
+
+    // This needs to be done after all the subscribers are set up
+    this.showAddAssignee$.next(false);
   }
 
   ngOnDestroy():void {
@@ -105,40 +188,6 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
     this.calendar.resizeObs?.disconnect();
   }
 
-  public calendarResourcesFunction(
-    fetchInfo:{ start:Date, end:Date, timeZone:string },
-    successCallback:(events:EventInput[]) => void,
-    failureCallback:(error:unknown) => void,
-  ):void|PromiseLike<EventInput[]> {
-    this
-      .calendar
-      .currentWorkPackages$
-      .toPromise()
-      .then((workPackages:WorkPackageCollectionResource) => {
-        const resources = this.mapToCalendarResources(workPackages.elements);
-        successCallback(resources);
-      })
-      .catch(failureCallback);
-  }
-
-  public calendarEventsFunction(
-    fetchInfo:{ start:Date, end:Date, timeZone:string },
-    successCallback:(events:EventInput[]) => void,
-    failureCallback:(error:unknown) => void,
-  ):void|PromiseLike<EventInput[]> {
-    this
-      .calendar
-      .currentWorkPackages$
-      .toPromise()
-      .then((workPackages:WorkPackageCollectionResource) => {
-        const events = this.mapToCalendarEvents(workPackages.elements);
-        successCallback(events);
-      })
-      .catch(failureCallback);
-
-    this.calendar.updateTimeframe(fetchInfo, this.projectIdentifier);
-  }
-
   private initializeCalendar() {
     void this.configuration.initialized
       .then(() => {
@@ -174,7 +223,8 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
               },
             },
             events: this.calendarEventsFunction.bind(this) as unknown,
-            resources: this.calendarResourcesFunction.bind(this) as unknown,
+            resources: [],
+            eventClick: this.openSplitView.bind(this) as unknown,
             resourceLabelContent: (data:ResourceLabelContentArg) => this.renderTemplate(this.resourceContent, data.resource.id, data),
             resourceLabelWillUnmount: (data:ResourceLabelContentArg) => this.unrenderTemplate(data.resource.id),
           } as CalendarOptions),
@@ -182,6 +232,24 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
       });
   }
 
+  public calendarEventsFunction(
+    fetchInfo:{ start:Date, end:Date, timeZone:string },
+    successCallback:(events:EventInput[]) => void,
+    failureCallback:(error:unknown) => void,
+  ):void|PromiseLike<EventInput[]> {
+    this
+      .calendar
+      .currentWorkPackages$
+      .toPromise()
+      .then((workPackages:WorkPackageCollectionResource) => {
+        const events = this.mapToCalendarEvents(workPackages.elements);
+        successCallback(events);
+      })
+      .catch(failureCallback);
+
+    this.calendar.updateTimeframe(fetchInfo, this.projectIdentifier);
+  }
+
   renderTemplate(template:TemplateRef<unknown>, id:string, data:ResourceLabelContentArg):{ domNodes:unknown[] } {
     const ref = this.viewLookup.getView(template, id, data);
     return { domNodes: ref.rootNodes };
@@ -191,18 +259,50 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
     this.viewLookup.destroyView(id);
   }
 
-  private setupWorkPackagesListener():void {
-    this.calendar.workPackagesListener$(() => {
-      this.renderCurrent();
-    });
+  public showAssigneeAddRow():void {
+    this.showAddAssignee$.next(true);
+    this.ucCalendar.getApi().refetchEvents();
   }
 
-  /**
-   * Renders the currently loaded set of items
-   */
-  private renderCurrent() {
-    this.ucCalendar.getApi().refetchEvents();
-    this.ucCalendar.getApi().refetchResources();
+  public addAssignee(principal:HalResource):void {
+    this.showAddAssignee$.next(false);
+
+    const modifyFilter = (assigneeFilter:QueryFilterInstanceResource) => {
+      // eslint-disable-next-line no-param-reassign
+      assigneeFilter.values = [
+        ...assigneeFilter.values as HalResource[],
+        principal,
+      ];
+    };
+
+    if (this.wpTableFilters.findIndex('assignee') === -1) {
+      // Replace actually also instantiates if it does not exist, which is handy here
+      this.wpTableFilters.replace('assignee', modifyFilter.bind(this));
+    } else {
+      this.wpTableFilters.modify('assignee', modifyFilter.bind(this));
+    }
+  }
+
+  public removeAssignee(href:string):void {
+    const numberOfAssignees = this.wpTableFilters.find('assignee')?.values?.length;
+    if (numberOfAssignees && numberOfAssignees <= 1) {
+      this.wpTableFilters.remove('assignee');
+    } else {
+      this.wpTableFilters.modify('assignee', (assigneeFilter:QueryFilterInstanceResource) => {
+        // eslint-disable-next-line no-param-reassign
+        assigneeFilter.values = (assigneeFilter.values as HalResource[])
+          .filter((filterValue) => filterValue.href !== href);
+      });
+    }
+  }
+
+  private openSplitView(event:EventClickArg):void {
+    const workPackage = event.event.extendedProps.workPackage as WorkPackageResource;
+
+    void this.$state.go(
+      `${splitViewRoute(this.$state)}.tabs`,
+      { workPackageId: workPackage.id, tabIdentifier: 'overview' },
+    );
   }
 
   private mapToCalendarEvents(workPackages:WorkPackageResource[]):EventInput[] {
@@ -231,23 +331,4 @@ export class TeamPlannerComponent extends UntilDestroyedMixin implements OnInit,
       })
       .filter((event) => !!event) as EventInput[];
   }
-
-  private mapToCalendarResources(workPackages:WorkPackageResource[]) {
-    const resources:{ id:string, title:string, user:HalResource }[] = [];
-
-    workPackages.forEach((workPackage:WorkPackageResource) => {
-      const assignee = workPackage.assignee as HalResource|undefined;
-      if (!assignee) {
-        return;
-      }
-
-      resources.push({
-        id: assignee.href as string,
-        title: assignee.name,
-        user: assignee,
-      });
-    });
-
-    return resources;
-  }
 }
diff --git a/frontend/src/app/features/team-planner/team-planner/planner/tp-assignee.sass b/frontend/src/app/features/team-planner/team-planner/planner/tp-assignee.sass
new file mode 100644
index 0000000000..da218add5e
--- /dev/null
+++ b/frontend/src/app/features/team-planner/team-planner/planner/tp-assignee.sass
@@ -0,0 +1,30 @@
+.tp-assignee
+  display: flex
+  max-width: 100%
+
+  &--principal
+    max-width: 100%
+    min-width: 0 // See: https://css-tricks.com/flexbox-truncated-text/
+    flex-grow: 1
+    flex-shrink: 1
+    
+  &--remove
+    background: white
+    border-radius: 50%
+    margin: 0
+    padding: 0
+    padding-left: 0.5rem
+    flex-grow: 0
+    flex-shrink: 0
+    width: 0
+    border: 0
+    background: transparent
+    cursor: normal
+    pointer-events: none
+    opacity: 0
+
+  @at-root &--remove:focus, &:hover &--remove, #{$spec-active-selector} &--remove
+    width: unset
+    cursor: pointer
+    opacity: 1
+    pointer-events: all
diff --git a/frontend/src/app/features/team-planner/team-planner/team-planner.module.ts b/frontend/src/app/features/team-planner/team-planner/team-planner.module.ts
index 7cf7a65894..05e9b073d4 100644
--- a/frontend/src/app/features/team-planner/team-planner/team-planner.module.ts
+++ b/frontend/src/app/features/team-planner/team-planner/team-planner.module.ts
@@ -4,10 +4,12 @@ import { UIRouterModule } from '@uirouter/angular';
 import { DynamicModule } from 'ng-dynamic-component';
 import { FullCalendarModule } from '@fullcalendar/angular';
 import { IconModule } from 'core-app/shared/components/icon/icon.module';
+import { OpenprojectAutocompleterModule } from 'core-app/shared/components/autocompleter/openproject-autocompleter.module';
 import { OpenprojectPrincipalRenderingModule } from 'core-app/shared/components/principal/principal-rendering.module';
 import { OpenprojectWorkPackagesModule } from 'core-app/features/work-packages/openproject-work-packages.module';
 import { TEAM_PLANNER_ROUTES } from 'core-app/features/team-planner/team-planner/team-planner.routes';
 import { TeamPlannerComponent } from 'core-app/features/team-planner/team-planner/planner/team-planner.component';
+import { AddAssigneeComponent } from 'core-app/features/team-planner/team-planner/planner/add-assignee.component';
 import { TeamPlannerPageComponent } from 'core-app/features/team-planner/team-planner/page/team-planner-page.component';
 import { OPSharedModule } from 'core-app/shared/shared.module';
 
@@ -15,6 +17,7 @@ import { OPSharedModule } from 'core-app/shared/shared.module';
   declarations: [
     TeamPlannerComponent,
     TeamPlannerPageComponent,
+    AddAssigneeComponent,
   ],
   imports: [
     OPSharedModule,
@@ -28,6 +31,8 @@ import { OPSharedModule } from 'core-app/shared/shared.module';
     OpenprojectPrincipalRenderingModule,
     OpenprojectWorkPackagesModule,
     FullCalendarModule,
+    // Autocompleters
+    OpenprojectAutocompleterModule,
   ],
 })
 export class TeamPlannerModule {}
diff --git a/frontend/src/app/features/work-packages/routing/wp-view-base/state/wp-single-view.query.ts b/frontend/src/app/features/work-packages/routing/wp-view-base/state/wp-single-view.query.ts
index 856be08025..6af224b645 100644
--- a/frontend/src/app/features/work-packages/routing/wp-view-base/state/wp-single-view.query.ts
+++ b/frontend/src/app/features/work-packages/routing/wp-view-base/state/wp-single-view.query.ts
@@ -19,7 +19,7 @@ export class WpSingleViewQuery extends Query<WpSingleViewState> {
     this.resourceService.query.select(),
   ]).pipe(
     filter((filters) => filters.length > 0),
-    map(([filters, state]) => selectCollectionAsEntities$<InAppNotification>(this.resourceService, state, { filters })),
+    map(([filters]) => selectCollectionAsEntities$<InAppNotification>(this.resourceService, { filters })),
   );
 
   selectNotificationsCount$ = this
diff --git a/frontend/src/app/shared/components/autocompleter/members-autocompleter/members-autocompleter.component.ts b/frontend/src/app/shared/components/autocompleter/members-autocompleter/members-autocompleter.component.ts
index da43b7d005..eeba0894b1 100644
--- a/frontend/src/app/shared/components/autocompleter/members-autocompleter/members-autocompleter.component.ts
+++ b/frontend/src/app/shared/components/autocompleter/members-autocompleter/members-autocompleter.component.ts
@@ -4,7 +4,7 @@ import { HttpClient, HttpParams } from '@angular/common/http';
 import { Component } from '@angular/core';
 import { PathHelperService } from 'core-app/core/path-helper/path-helper.service';
 import {
-  UserAutocompleteItem,
+  IUserAutocompleteItem,
   UserAutocompleterComponent,
 } from 'core-app/shared/components/autocompleter/user-autocompleter/user-autocompleter.component';
 import { URLParamsEncoder } from 'core-app/features/hal/services/url-params-encoder';
@@ -20,9 +20,9 @@ export class MembersAutocompleterComponent extends UserAutocompleterComponent {
 
   @InjectField() pathHelper:PathHelperService;
 
-  protected getAvailableUsers(url:string, searchTerm:any):Observable<UserAutocompleteItem[]> {
+  protected getAvailableUsers(url:string, searchTerm:any):Observable<IUserAutocompleteItem[]> {
     return this.http
-      .get<UserAutocompleteItem[]>(url,
+      .get<IUserAutocompleteItem[]>(url,
       {
         params: new HttpParams({ encoder: new URLParamsEncoder(), fromObject: { q: searchTerm } }),
         responseType: 'json',
diff --git a/frontend/src/app/shared/components/autocompleter/user-autocompleter/user-autocompleter.component.ts b/frontend/src/app/shared/components/autocompleter/user-autocompleter/user-autocompleter.component.ts
index 2605eb12cc..c02bc58e70 100644
--- a/frontend/src/app/shared/components/autocompleter/user-autocompleter/user-autocompleter.component.ts
+++ b/frontend/src/app/shared/components/autocompleter/user-autocompleter/user-autocompleter.component.ts
@@ -46,10 +46,11 @@ import { ApiV3FilterBuilder, FilterOperator } from 'core-app/shared/helpers/api-
 
 export const usersAutocompleterSelector = 'user-autocompleter';
 
-export interface UserAutocompleteItem {
+export interface IUserAutocompleteItem {
   name:string;
   id:string|null;
   href:string|null;
+  avatar:string|null;
 }
 
 @Component({
@@ -61,7 +62,7 @@ export class UserAutocompleterComponent implements OnInit {
 
   @ViewChild(NgSelectComponent, { static: true }) public ngSelectComponent:NgSelectComponent;
 
-  @Output() public onChange = new EventEmitter<void>();
+  @Output() public onChange = new EventEmitter<IUserAutocompleteItem>();
 
   @Input() public clearAfterSelection = false;
 
@@ -80,7 +81,7 @@ export class UserAutocompleterComponent implements OnInit {
   private updateInputField:HTMLInputElement|undefined;
 
   /** Keep a switchmap for search term and loading state */
-  public requests = new DebouncedRequestSwitchmap<string, UserAutocompleteItem>(
+  public requests = new DebouncedRequestSwitchmap<string, IUserAutocompleteItem>(
     (searchTerm:string) => this.getAvailableUsers(this.url, searchTerm),
     errorNotificationHandler(this.halNotification),
   );
@@ -157,7 +158,7 @@ export class UserAutocompleterComponent implements OnInit {
     }
   }
 
-  protected getAvailableUsers(url:string, searchTerm:any):Observable<UserAutocompleteItem[]> {
+  protected getAvailableUsers(url:string, searchTerm:any):Observable<IUserAutocompleteItem[]> {
     // Need to clone the filters to not add additional filters on every
     // search term being processed.
     const searchFilters = this.inputFilters.clone();
diff --git a/frontend/src/app/shared/components/principal/principal-helper.ts b/frontend/src/app/shared/components/principal/principal-helper.ts
index 124e61822c..bdb345a717 100644
--- a/frontend/src/app/shared/components/principal/principal-helper.ts
+++ b/frontend/src/app/shared/components/principal/principal-helper.ts
@@ -26,10 +26,30 @@
 // See COPYRIGHT and LICENSE files for more details.
 //++
 
+import { PrincipalLike } from 'core-app/shared/components/principal/principal-types';
+import { Principal } from 'core-app/core/state/principals/principal.model';
+
 export namespace PrincipalHelper {
   export type PrincipalType = 'user'|'placeholder_user'|'group';
   export type PrincipalPluralType = 'users'|'placeholder_users'|'groups';
 
+  /*
+   * This function is a helper that wraps around the old HalResource based principal type and the new interface based one.
+   *
+   * TODO: Remove old HalResource stuff :P
+   */
+  export function hrefFromPrincipal(p:Principal|PrincipalLike):string {
+    if ((p as PrincipalLike).href) {
+      return (p as PrincipalLike).href || '';
+    }
+
+    if ((p as Principal)._links) {
+      const self = (p as Principal)._links.self as HalSourceLink;
+      return self.href || '';
+    }
+
+    return '';
+  }
   export function typeFromHref(href:string):PrincipalType|null {
     const match = /\/(user|group|placeholder_user)s\/\d+$/.exec(href);
 
diff --git a/frontend/src/app/shared/components/principal/principal-renderer.service.ts b/frontend/src/app/shared/components/principal/principal-renderer.service.ts
index 335720907c..91c8bf96c2 100644
--- a/frontend/src/app/shared/components/principal/principal-renderer.service.ts
+++ b/frontend/src/app/shared/components/principal/principal-renderer.service.ts
@@ -3,6 +3,7 @@ import { PathHelperService } from 'core-app/core/path-helper/path-helper.service
 import { ColorsService } from 'core-app/shared/components/colors/colors.service';
 import { APIV3Service } from 'core-app/core/apiv3/api-v3.service';
 import idFromLink from 'core-app/features/hal/helpers/id-from-link';
+import { Principal } from 'core-app/core/state/principals/principal.model';
 import { PrincipalLike } from './principal-types';
 import { PrincipalHelper } from './principal-helper';
 import PrincipalType = PrincipalHelper.PrincipalType;
@@ -29,7 +30,7 @@ export class PrincipalRendererService {
 
   renderMultiple(
     container:HTMLElement,
-    users:PrincipalLike[],
+    users:PrincipalLike[]|Principal[],
     name:NameOptions = { hide: false, link: false },
     avatar:AvatarOptions = { hide: false, size: 'default' },
     multiLine = false,
@@ -59,12 +60,12 @@ export class PrincipalRendererService {
 
   render(
     container:HTMLElement,
-    principal:PrincipalLike,
+    principal:PrincipalLike|Principal,
     name:NameOptions = { hide: false, link: true },
     avatar:AvatarOptions = { hide: false, size: 'default' },
   ):void {
     container.classList.add('op-principal');
-    const type = PrincipalHelper.typeFromHref(principal.href || '') as PrincipalType;
+    const type = PrincipalHelper.typeFromHref(PrincipalHelper.hrefFromPrincipal(principal)) as PrincipalType;
 
     if (!avatar.hide) {
       const el = this.renderAvatar(principal, avatar, type);
@@ -78,7 +79,7 @@ export class PrincipalRendererService {
   }
 
   private renderAvatar(
-    principal:PrincipalLike,
+    principal:PrincipalLike|Principal,
     options:AvatarOptions,
     type:PrincipalType,
   ) {
@@ -86,6 +87,7 @@ export class PrincipalRendererService {
     const colorCode = this.colors.toHsl(principal.name);
 
     const fallback = document.createElement('div');
+    fallback.classList.add('op-principal--avatar');
     fallback.classList.add('op-avatar');
     fallback.classList.add(`op-avatar_${options.size}`);
     fallback.classList.add(`op-avatar_${type.replace('_', '-')}`);
@@ -108,7 +110,7 @@ export class PrincipalRendererService {
     return fallback;
   }
 
-  private renderUserAvatar(principal:PrincipalLike, fallback:HTMLElement, options:AvatarOptions):void {
+  private renderUserAvatar(principal:PrincipalLike|Principal, fallback:HTMLElement, options:AvatarOptions):void {
     const url = this.userAvatarUrl(principal);
 
     if (!url) {
@@ -128,12 +130,12 @@ export class PrincipalRendererService {
     };
   }
 
-  private userAvatarUrl(principal:PrincipalLike):string|null {
-    const id = principal.id || idFromLink(principal.href || '');
+  private userAvatarUrl(principal:PrincipalLike|Principal):string|null {
+    const id = principal.id || idFromLink(PrincipalHelper.hrefFromPrincipal(principal));
     return id ? this.apiV3Service.users.id(id).avatar.toString() : null;
   }
 
-  private renderName(principal:PrincipalLike, type:PrincipalType, asLink = true) {
+  private renderName(principal:PrincipalLike|Principal, type:PrincipalType, asLink = true) {
     if (asLink) {
       const link = document.createElement('a');
       link.textContent = principal.name;
@@ -150,8 +152,9 @@ export class PrincipalRendererService {
     return span;
   }
 
-  private principalURL(principal:PrincipalLike, type:PrincipalType):string {
-    const id = principal.id || (principal.href ? idFromLink(principal.href) : '');
+  private principalURL(principal:PrincipalLike|Principal, type:PrincipalType):string {
+    const href = PrincipalHelper.hrefFromPrincipal(principal);
+    const id = principal.id || (href ? idFromLink(href) : '');
 
     switch (type) {
       case 'group':
diff --git a/frontend/src/assets/sass/_helpers.sass b/frontend/src/assets/sass/_helpers.sass
index 0be97a8b3e..fad5fa09b5 100644
--- a/frontend/src/assets/sass/_helpers.sass
+++ b/frontend/src/assets/sass/_helpers.sass
@@ -4,4 +4,5 @@
  * importing these helpers!
  */
 @import "~global_styles/openproject/_mixins"
+@import "~global_styles/openproject/_variables"
 @import "~global_styles/content/drag_and_drop"
diff --git a/frontend/src/global_styles/content/_principal.sass b/frontend/src/global_styles/content/_principal.sass
index 9454fb2258..f65c99f0bb 100644
--- a/frontend/src/global_styles/content/_principal.sass
+++ b/frontend/src/global_styles/content/_principal.sass
@@ -1,4 +1,17 @@
 .op-principal
+  display: inline-flex
+  align-items: center
+
+  &--avatar
+    flex-grow: 0
+    flex-shrink: 0
+
+  &--name
+    @include text-shortener 
+    flex-grow: 1
+    flex-shrink: 1
+    min-width: 0 // See: https://css-tricks.com/flexbox-truncated-text/
+
   &--multi-line
     display: block
     margin-bottom: 2px
diff --git a/frontend/src/global_styles/content/modules/_team_planner.sass b/frontend/src/global_styles/content/modules/_team_planner.sass
index 6eea31f8f1..096967199c 100644
--- a/frontend/src/global_styles/content/modules/_team_planner.sass
+++ b/frontend/src/global_styles/content/modules/_team_planner.sass
@@ -1,3 +1,5 @@
+@import "~app/features/team-planner/team-planner/planner/tp-assignee"
+
 .router--team-planner
   #content
     height: 100%
diff --git a/frontend/src/global_styles/openproject/_mixins.sass b/frontend/src/global_styles/openproject/_mixins.sass
index 22c75b9980..e0ce0b5a14 100644
--- a/frontend/src/global_styles/openproject/_mixins.sass
+++ b/frontend/src/global_styles/openproject/_mixins.sass
@@ -273,3 +273,4 @@ $scrollbar-size: 10px
 
   &:focus
     right: 0
+
diff --git a/frontend/src/global_styles/openproject/_variables.sass b/frontend/src/global_styles/openproject/_variables.sass
new file mode 100644
index 0000000000..b870aca162
--- /dev/null
+++ b/frontend/src/global_styles/openproject/_variables.sass
@@ -0,0 +1,2 @@
+// A selector that checks whether we are running in a test environment
+$spec-active-selector: '.env-test'
diff --git a/modules/team_planner/config/locales/js-en.yml b/modules/team_planner/config/locales/js-en.yml
index 0bf7129f8e..8d51d11f8a 100644
--- a/modules/team_planner/config/locales/js-en.yml
+++ b/modules/team_planner/config/locales/js-en.yml
@@ -5,3 +5,5 @@ en:
       title: 'Team planner'
       unsaved_title: 'Unnamed team planner'
       label_assignee_plural: 'Assignees'
+      add_assignee: 'Add assignee'
+      remove_assignee: 'Remove assignee'
diff --git a/modules/team_planner/spec/features/team_planner_spec.rb b/modules/team_planner/spec/features/team_planner_spec.rb
index e4585eeb20..ffa98ff779 100644
--- a/modules/team_planner/spec/features/team_planner_spec.rb
+++ b/modules/team_planner/spec/features/team_planner_spec.rb
@@ -73,7 +73,16 @@ describe 'Team planner', type: :feature, js: true do
   end
 
   context 'with an assigned work package' do
-    let!(:other_user) { FactoryBot.create :user, firstname: 'Other', lastname: 'User' }
+    let!(:other_user) do
+      FactoryBot.create :user,
+                        firstname: 'Other',
+                        lastname: 'User',
+                        member_in_project: project,
+                        member_with_permissions: %w[
+                          view_work_packages edit_work_packages view_team_planner manage_team_planner
+                        ]
+    end
+    let!(:user_outside_project) { FactoryBot.create :user, firstname: 'Not', lastname: 'In Project' }
     let(:type_task) { FactoryBot.create :type_task }
     let(:type_bug) { FactoryBot.create :type_bug }
 
@@ -115,6 +124,21 @@ describe 'Team planner', type: :feature, js: true do
 
       team_planner.title
 
+      team_planner.expect_assignee(user, present: false)
+      team_planner.expect_assignee(other_user, present: false)
+
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.select_user_to_add user.name
+      end
+
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.select_user_to_add other_user.name
+      end
+
       team_planner.expect_assignee user
       team_planner.expect_assignee other_user
 
@@ -135,7 +159,8 @@ describe 'Team planner', type: :feature, js: true do
       filters.expect_filter_by('Type', 'is', [type_task.name])
       filters.expect_filter_count("2")
 
-      team_planner.expect_assignee(user, present: false)
+      team_planner.expect_assignee(user, present: true)
+      team_planner.expect_assignee(other_user, present: true)
 
       team_planner.within_lane(other_user) do
         team_planner.expect_event other_task
@@ -147,8 +172,79 @@ describe 'Team planner', type: :feature, js: true do
       split_view.edit_field(:type).update(type_bug)
       split_view.expect_and_dismiss_toaster(message: "Successful update.")
 
+      team_planner.expect_assignee(user, present: true)
+      team_planner.expect_assignee(other_user, present: true)
+    end
+
+    it 'can add and remove assignees' do
+      team_planner.visit!
+
       team_planner.expect_assignee(user, present: false)
       team_planner.expect_assignee(other_user, present: false)
+      
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.select_user_to_add user.name
+      end
+
+      team_planner.expect_assignee(user)
+      team_planner.expect_assignee(other_user, present: false)
+      
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.select_user_to_add other_user.name
+      end
+
+      team_planner.expect_assignee(user)
+      team_planner.expect_assignee(other_user)
+
+      team_planner.remove_assignee(user)
+
+      team_planner.expect_assignee(user, present: false)
+      team_planner.expect_assignee(other_user)
+
+      team_planner.remove_assignee(other_user)
+
+      team_planner.expect_assignee(user, present: false)
+      team_planner.expect_assignee(other_user, present: false)
+
+      # Try one more time to make sure deleting the full filter didn't kill the functionality
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.select_user_to_add user.name
+      end
+
+      team_planner.expect_assignee(user)
+      team_planner.expect_assignee(other_user, present: false)
+    end
+
+    it 'filters possible assignees correctly' do
+      team_planner.visit!
+
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.search_user_to_add user_outside_project.name
+      end
+
+      expect(page).to have_selector('.ng-option-disabled', text: "No items found")
+      
+      retry_block do
+        team_planner.select_user_to_add user.name
+      end
+      
+      team_planner.expect_assignee(user)
+
+      retry_block do
+        team_planner.click_add_user
+        page.find('[data-qa-selector="tp-add-assignee"] input')
+        team_planner.search_user_to_add user.name
+      end
+
+      expect(page).to have_selector('.ng-option-disabled', text: "No items found")
     end
   end
 end
diff --git a/modules/team_planner/spec/support/pages/team_planner.rb b/modules/team_planner/spec/support/pages/team_planner.rb
index 0f3b5546e1..7bcb34e500 100644
--- a/modules/team_planner/spec/support/pages/team_planner.rb
+++ b/modules/team_planner/spec/support/pages/team_planner.rb
@@ -30,6 +30,8 @@ require 'support/pages/page'
 
 module Pages
   class TeamPlanner < ::Pages::Page
+    include ::Components::NgSelectAutocompleteHelpers
+
     attr_reader :project,
                 :filters
 
@@ -53,6 +55,10 @@ module Pages
       expect(page).to have_conditional_selector(present, '.fc-resource', text: name, wait: 10)
     end
 
+    def remove_assignee(user)
+      page.find(%([data-qa-remove-assignee="#{user.id}"])).click
+    end
+
     def within_lane(user, &block)
       raise ArgumentError.new("Expected instance of principal") unless user.is_a?(Principal)
 
@@ -73,5 +79,21 @@ module Pages
 
       ::Pages::SplitWorkPackage.new(work_package, project)
     end
+
+    def click_add_user
+      page.find('[data-qa-selector="tp-assignee-add-button"]').click
+    end
+
+    def select_user_to_add(name)
+      select_autocomplete page.find('[data-qa-selector="tp-add-assignee"]'),
+                          query: name,
+                          results_selector: 'body'
+    end
+
+    def search_user_to_add(name)
+      search_autocomplete page.find('[data-qa-selector="tp-add-assignee"]'),
+                          query: name,
+                          results_selector: 'body'
+    end
   end
 end