Mensa seems to have a pretty bad reputation in the US. I never had much opinion about and didn't know any Mensa members until my sister joined. She meets loads of interesting people there. They have game weekends where they play all sorts of interesting board games, but also in other activities it's nice to be around people who think a bit like you do.

I've never felt the need to join Mensa because I studied CS and AI, and all my friends from there are probably about as smart as I am, so I already have plenty of people to relate to, but she was always very interested in sport, which probably attracts a different crowd (though she's working in IT now).