As Space administrator, you need to give access to different user-groups by adding them under "groups" section.

To produce the bug :

1) Click on edit permissions button
2) Under Groups section, enter (existing) "userid" instead of user group.
3) This userid will be get added as user-group; which is confusing.
4) Also I have seen that you can add users to this user group (this I have tested long back; not sure now!)

I just gone through list open bugs; couldn't find one related to this.

Let me know if you have questions; I can add screen shot if necessary