I went to their meetings a couple times by invitation, and it never really "clicked" - for many of their members, their MENSA membership and related activities are a big part of their identity and their life, and I wasn't able to connect. I heard the same thing from a couple of friends who also met their entrance criteria, and there's certainly a lot of selection bias. Which is fine, but it's not representative by any means.
That's a bit too loose analogy. A more precise is like joining because one is tall.
While the principle still somewhat holds, the difference is in the consequences. One may join Mensa because they're likely to find people with similar tendencies/interests, as much as tall people may like certain sports.
The selection bias works the other way as well: people who are happy with their Mensa social group get their social contact from that and don't bother hanging out on HN.
But I thought membership was well worth it. Discussions about random arcane topics, meetings and conferences, contacts when traveling, dating, a fairly open and tolerant social circle, etc.
I used to go social dancing and the purpose of that group of people was to dance, and they would have various events all about dancing or improving your dancing. If you join a social justice or politically minded group, I imagine your purpose is to engage in activities to further your ideals. But to join a club where there are other smart people... your goal is to what? Share interesting puzzles with each other? :D Try to make other people smarter?
I very rarely discuss IQ since so many people are hostile to it. A high IQ makes you different. Sometimes it's subtle, like as an adult you'll typically have different interests compared to a lot of peers, and other times it's far less subtle, like when you're a school aged kid and you're always made to feel different (FYI most school aged kids don't want to feel different).
With smart people, I suppose any topic is on the table. It's not specific enough for me.
Most topics here might interest people for professional reasons. But they include things like "High IQs are associated with mental and physical disorders" too. People love to debate politics. And "Is the Ship Still Stuck?"[1] is the top article right now.
That's the case for many people that join a club, though, be it chess, functional programming, golf, gardening, or what have you, and happens naturally if that's where you spend a good chunk of your free time.
At least that was the only real benefit I remember seeing in my limited research years ago.