Your group should be open for joining
After a group join via widget postMessage with following data will be sent to your page:
ok_join$__okGroup1
You can track event on page with widget by the following javascript code:
function listenForJoin() { if (window.addEventListener) { window.addEventListener('message', onJoin, false); } else { window.attachEvent('onmessage', onJoin); } } function onJoin(e) { var args = e.data.split("$"); if (args[0] == "ok_join") { alert(args[1]); } } listenForJoin();