cannot add users if there are custom plugin permissions

[cnb]

If there's a plugin that uses custom permissions (like News Manager 3.3+) you can edit existing users but cannot add new ones - the MU plugin crashes.
Reported here:
http://get-simple.info/forums/showthread.php?tid=1549&pid=59998#pid59998

It seems it's just because the plugin doesn't have this in function mmAddUser():

global $perm;