Comment 7 for bug 18351

The current workaround is:
1. In users-admin, click "Manage Groups"
2. Select the desired group, and click Properties
3. Check the box next to the user name you want to add to the group

There's still another problem, though: even Manage Groups doesn't list all of the group names on the system. 'video' and 'disk', for example, aren't showing on mine.