The easy solution is the categories that are read by (and maybe reply by) all. Then everyone can see everything, and it’ll be obvious to those who aren’t premium members what they are missing.
I don’t see the problem with letting people know who is in the premium groups, but I think that you could use css to major those members not obvious to staff. I think that the group membership is in the json load, so you could target it, though it would be available to those who look at the json.