Could you try to describe a little more what you want to accomplish?
Which admin operations on a user do you want to be able to grant selectively?
I don't see what kind of sense a membership relation between two users would make - why should a user be a member of another user? Maybe you would rather want to grant explicit permissions on the user object, but these won't be reflected in the admin pages I am afraid, because they just check for subsite or site-wide admin.
