When I discovered my 9th grad physics teacher was in Mensa I vowed to never join. And I kept that vow.
Which, honestly, was pretty easy. I suppose they're harmless, but I sort of think of Mensa and big-L Libertarians in the same sense. We have some things in common, but they take themselves way too seriously and they want you to hew a little too close to whatever bizarre line they are walking.