code: public void sendMessage(String message, boolean push, boolean persist,
                              final Respoke.TaskCompletionListener completionListener) {
          if (isJoined()) {
              if ((null != groupID) && (groupID.length() > 0)) {
                  RespokeClient client = clientReference.get();
                  if (null != client) {
  
                      JSONObject data = new JSONObject();
  
                      try {
                          data.put("endpointId", client.getEndpointID());
                          data.put("message", message);
                          data.put("push", push);
                          data.put("persist", persist);
                      } catch (JSONException e) {
                          Respoke.postTaskError(completionListener, "Unable to encode message");
                          return;
                      }
  
                      String urlEndpoint = "/v1/channels/" + groupID + "/publish/";
                      signalingChannel.sendRESTMessage("post", urlEndpoint, data, new RespokeSignalingChannel.RESTListener() {
                          @Override
                          public void onSuccess(Object response) {
                              Respoke.postTaskSuccess(completionListener);
                          }
  
                          @Override
                          public void onError(final String errorMessage) {
                              Respoke.postTaskError(completionListener, errorMessage);
                          }
                      });
                  } else {
                      Respoke.postTaskError(completionListener, "There was an internal error processing this request.");
                  }
              } else {
                  Respoke.postTaskError(completionListener, "Group name must be specified");
              }
          } else {
              Respoke.postTaskError(completionListener, "Not a member of this group anymore.");
          }
      }
docs: Send a message to the entire group.
  
  @param message             The message to send
  @param push                A flag indicating if a push notification should be sent for this message
  @param persist             A flag indicating if history should be maintained for this message.
  @param completionListener  A listener to receive a notification on the success of the asynchronous operation