Groups should be able to contain other groups

We have a very large user base (thousands of users).

It would be good if a group could contain a reference to other groups since this makes maintaining groups much less tedious.

For example:

Group A: User1, User2

Group B: User65, User 98

Group C: Group A, Group B, User 142

Group C is in a sense "dyanmic" so that if another user is added to Group A then Group C is effectively updated too (i.e. I'm not request the ability to have a "one time copy" of users into a new group).