[Marco van Herwaarden]

Something like:

PHP Code:

$vbulletin->input->clean_gpc('r', 'invitation', TYPE_NOHTML);
 
if ($vbulletin->GPC['invitation'])
{ 
  $invitegroupid = $db->query_first_slave("SELECT NewusergroupID FROM " . TABLE_PREFIX . "vbinvitationcode WHERE invitation = '" .  $db->escape_string($vbulletin->GPC['InvitationCODE'] . "'");
  $userdata->set('usergroupid', $invitedgroupid);  
}
else
{
  $userdata->set('usergroupid', $newusergroupid);
} 


But just guessing. If you need more assistence, please also post the error message.