I joined 20 years ago out of curiousity if I could pass. I did and paid for a membership for a year because why not, but i never did anything with it and never renewed it.
The one thing I might have liked is the local meetings. Our local chapter had a website where they talked about the meetings. They were informal, just dinner and chatting in a private room at a restaurant, and you could learn a little about the people from the website too. It seemed to be heavy on scientists and engineers. This is near a national labratory so that made sense. I would have liked to have more friends like that, still would. But, for all I know, mensa scientists and engineers might be the worst scientists and engineers. They also appeared to be mostly over 60 and I was 20 at the time.
There are discounts on certain things according to their website https://www.us.mensa.org/shop/benefits-and-services/