Other than AAA and the Phi Kappa Phi honor fraternity, I don't beloing to any other organizations presently.
In years past, I did belong (and do loads of volunteer work for) the Metrolina AIDS Project, Theatre Charlotte, the Charlotte Atari Users Group, and all the professional teachers' organizations for my local, state, and national groups (NEA, SCEA, LCEA).
The most fun social group I ever belonged to was the Charlotte Broach Society. It was ten gay men (some single, some partnered) who had holiday event parties, sponsored fund raising events for charities, and had a dinner at one of the group's homes each month. As people broke up, moved away, etc. the group eventually disbanded, but for about two years, it was THE most fun.