#1) Try adding this to the top of the page:
php:
<?php
ini_set("display_errors",1);
?>
That should at least give you an error message instead of a blank page so we can have something to go on.
#2) Just save the group value with MM_Usergroup and then all of the old user group server behaviors should work just like they used to. You can do more complex things with Security Assist Groups and Rules, but if you are just trying to replicate the old functionality then this is the easiest solution.
Did this help? Tips are appreciated...
