code: public void addListener(final ParticipantsListener participantsListener) {
          if (participantsListener != null) {
              MessagingListener messagingListener = new MessagingListener() {
  
                  @Override
                  public void onParticipantAdded(ParticipantAddedEvent event) {
                      participantsListener.onParticipantAdded(event);
                  }
  
                  @Override
                  public void onParticipantUpdated(ParticipantUpdatedEvent event) {
                      participantsListener.onParticipantUpdated(event);
                  }
  
                  @Override
                  public void onParticipantRemoved(ParticipantRemovedEvent event) {
                      participantsListener.onParticipantRemoved(event);
                  }
              };
              participantsListeners.put(participantsListener, messagingListener);
              client.addListener(messagingListener);
          }
      }
docs: Registers listener for changes in participant list in conversations. Delivers participant added, updated and removed events.
  
  @param participantsListener Listener for changes in participant list in conversations.