This is very similar to the problem .LRN solves for clubs and various classes of members if I'm reading you properly, so you might look at the way they use groups and relsegs.
It's often easiest to make a relseg that mirrors the main group BTW. I'd done that for subsites where membership_rel includes everyone in the group (you subtype new rels for new user classes/roles) The cost of this duplication is light, you don't need new rows in the acs-rels table, just the relseg wrapper. The nice thing about doing it this way is that you can grab any subset of users, including the subset that contains all group members, using the same Tcl calls, which makes it a bit easier to generalize stuff and to avoid special-casing ordinary members vs. admins etc.
