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<? //Get member handler
$member_handler =& xoops_gethandler('member');
//Add user to group
$member_handler->addUserToGroup($groupid, $uid); ?>
to list all groups so you can add them in a form :
Class: XoopsFormSelectGroup
Method: XoopsFormSelectGroup($caption, $name, $include_anon=false, $value=null, $size=1, $multiple=false)
This file is in class/xoopsform
hope this helps you to hack your script