[Usability] Request for UI ideas: User/Group management

["sam" <spam2 debain org>]

Hi,

I am trying to desig a GUI for assigning permissions to specific groups 
of users. The GUI must make it possible to design a user/group hierarchy 
like this:

--------------------------
[Everyone]
\
 |- [Administrators]
 |    \- Samuel
 |- [Editors]
 |    |- Sarah
 |    \- Harry
 \- [Normal Users]
      |- Anonymous George
      |- Anonymous Jane
      \- [Friends]
          |- Cathy
          |- Jens
          \- Stefan
--------------------------
(I enclosed the group names in brackets, the "Everyone" group is mandatory)

Note that the following must be possible:

- Storing a user in multiple groups
- Storing a group in any another group, at infinite depth.

The permissions are assigned to groups or users, using inheritance from one
group to another. So collisions are possible in theory (what if a user is in a 
group that permits access to an object and also in a group that denies 
permission to the same object), but will be avoided by reporting them to 
the user when he tries to create such a situation.

I was trying to avoid using a tree view, but so far my ideas are few. I 
thought of using something like a tag system as opposed to a group 
system, but that makes the inheritance idea non-intuitive. Any ideas for 
a GUI?

Thanks,
-Samuel