code: public void join(String groupId, Boolean acceptRules) throws FlickrException {
  
          Map<String, Object> parameters = new HashMap<String, Object>();
          parameters.put("method", METHOD_JOIN);
          parameters.put("group_id", groupId);
          if (acceptRules != null) {
              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: Join a group as a public member.
  
  Note: if a group has rules - the client must display the rules to the user and the user must accept them prior to joining the group. The acceptRules
  parameter indicates that the user has accepted those rules.
  
  @param groupId
  - the id of the group to join
  @param acceptRules
  - if a group has rules, true indicates the user has accepted the rules
  
  @see <a href="http://www.flickr.com/services/api/flickr.groups.join.html">flickr.groups.join</a>