There is a FR that the user admin should be able to asign groups too:
http://project.geeklog.net/tracking/view.php?id=190[*1]
Now I have an additional idea. If I create a new group there should be a check box "make default for new users". So the group admin should be able to set one or more groups to default for new users.
E.g. the new group "football club supporters" with permission to upload pictures to the gallery should be default.