I think it's a good idea to define a specific group's subgroup. Not all of them are for posting though, so we'd have to be able to have a field or something to describe each individual one.
For example, my subgroups in both groups.io groups are simply the message archive from the equivalent group in Yahoo. So they are there solely for getting information, not for posting.
So if we can say, create our own description for the subgroup rather than have something like a drop-down box with selections, I think it would work out fine.