code: public void joinRequest(String groupId, String message, boolean acceptRules) throws FlickrException {
          Map<String, Object> parameters = new HashMap<String, Object>();
          parameters.put("method", METHOD_JOIN_REQUEST);
          parameters.put("group_id", groupId);
          parameters.put("message", message);
          parameters.put("accept_rules", acceptRules);
  
          Response response = transportAPI.post(transportAPI.getPath(), parameters, apiKey, sharedSecret);
          if (response.isError()) {
              throw new FlickrException(response.getErrorCode(), response.getErrorMessage());
          }
      }
docs: Request to join a group.
  
  Note: if a group has rules, the client must display the rules to the user and the user must accept them (which is indicated by passing a true value to
  acceptRules) prior to making the join request.
  
  @param groupId
  - groupId parameter
  @param message
  - (required) message to group administrator
  @param acceptRules
  - (required) parameter indicating user has accepted groups rules