<form [formGroup]="createInvitationForm" (ngSubmit)="createInvitation()">
    <mat-form-field class="full-width-input">
      <input matInput type="text" formControlName="invitationComment" placeholder="Send a comment to the group(s)." required>
      <mat-error *ngIf="createInvitationForm.get('invitationComment').hasError('required')">
        Invitation comment required
      </mat-error>
    </mat-form-field>
    <button mat-raised-button type="submit" color="primary" [disabled]="selectedGroups.length === 0 || !createInvitationForm.valid">Send invitation to selected group(s)</button>
  </form>

this.createInvitationForm.reset();