[kh99]

Around line 210 there's a section that starts:

// ############################### start add member ###############################
if ($_POST['do'] == 'addmember')
{

There's a hook register_addmember_process that happens just before the data is sent to the database and register_addmember_complete that happens nearer to the end, if you're looking for a hook to use. Note that if you have new members moderated or use email confirmation, the member will be in the moderated group and/or email won't be confirmed yet at that point.