'UserAddGroup': called when adding a group; return false to override
stock group addition.
$user: the user object that is to have a group added
-$group: the group to add
+&$group: the group to add, can be modified
'UserArrayFromResult': called when creating an UserArray object from a
database result
'UserRemoveGroup': called when removing a group; return false to override
stock group removal.
$user: the user object that is to have a group removed
-$group: the group to be removed
+&$group: the group to be removed, can be modified
'UserRights': After a user's group memberships are changed
$user : User object that was changed