Jomsocial Extra Field - plugins

Question

I am trying to create extra fields on the Jomsocial Groups Create New Group page, its suggested on the Jomsocial docs that the best way is to creat a plugin to do this.
As I have never created such a complex plugin do anyone have a working example to start with?
Here is the code that I have tried with
<?php
defined('_JEXEC') or die('Restricted access');
if (! class_exists ( 'plgSpbgrouppostcode' )) {
class plgSpbgrouppostcode extends JPlugin {
/**
* Method construct
*/
function plgSystemExample($subject, $config) {
parent::__construct($subject, $config);
// JPlugin::loadLanguage ( 'plg_system_example', JPATH_ADMINISTRATOR ); // only use if theres any language file
include_once( JPATH_ROOT .'/components/com_community/libraries/core.php' ); // loading the core library now
}
function onFormDisplay( $form_name )
{
/*
Add additional form elements at the bottom privacy page
*/
$elements = array();
if( $form_name == 'jsform-groups-forms' )
{
$obj = new CFormElement();
$obj->label = 'Labe1 1';
$obj->position = 'after';
$obj->html = '<input name="custom1" type="text">';
$elements[] = $obj;
$obj = new CFormElement();
$obj->label = 'Labe1 2';
$obj->position = 'after';
$obj->html = '<input name="custom2" type="text">';
$elements[] = $obj;
}
return $elements;