I've never found joining Mensa to be of interest to me, but have no objection to other people finding it of value. But then, I have GT siblings and extended family members, and a GT spouse and in-laws, so I already have access to an intellectual community
I did participate incidentally (we were in the same place, at the same time, and they invited me to hang out) in a couple of social gatherings years ago, when the local Mensa chapter had a standing lunch meeting at a restaurant owned by my SO's family. That particular group was mixed in gender, but not particularly in age (slanted toward the older end of middle aged, which, at the time, was quite a bit older than I). I think it's much like many social groups with a nominal theme--however people become members, the real purpose is basic human connection. The ostensible purpose is just an ice-breaker.
