code: public void addInvitationListener(WorkgroupInvitationListener invitationListener) {
          synchronized (invitationListeners) {
              if (!invitationListeners.contains(invitationListener)) {
                  invitationListeners.add(invitationListener);
              }
          }
      }
docs: Adds an invitation listener.
  
  @param invitationListener the invitation listener.