what societies can you join as a postgrad?

For the most part, from what I can tell, colleges don't limit club memberships to undergraduates only. We had a few postgrads in ours (an environmental club and an academic honor society) and it was no big deal. The clubs are primarily undergrads, though, just because the grad students seem to be busier...

Inquire at the university you're going to--most schools' websites have a list of clubs under "Student Union/Student Life" or some similar heading. The list usually has contact info, so you can just email them to get an idea.