code: public void getParticipants(@NonNull final String conversationId, @Nullable Callback<ComapiResult<List<Participant>>> callback) {
          adapter.adapt(getParticipants(conversationId), callback);
      }
docs: Returns observable to add a participant to.
  
  @param conversationId ID of a conversation to add a participant to.
  @param callback       Callback to deliver new session instance.