User group admins

[apeisa]

Group admin has the ability to add/edit users within a group.

[apeisa]

I am pretty sure that this doesn't need to be "core" feature and nothing to do with 1.0 release. What I don't know yet, is wheter we need to think this as something that core should be able to provide support (meaning, that it would be possible to build addition "group admin" module to give this feature).