I was a Mensa associate, and participated to evening meetings in two countries (evening meetings are only part of the activities).
You're assuming, with "people insecure enough to join an IQ-restricted club", that associates sit on a high horse due to their IQ. It was quite the opposite; the clubs are very friendly, and what I remember is actually, fun nights playing board and social games (it's amusing to think that a significant part of a "high IQ" club is games :)). Definitely, nobody would recognize it was a Mensa meeeting if they didn't know it already.
In my personal view, joining the club because of (relatively) high IQ was no different than joining, say, a chess club - meeting people who have something in common.
Needless to say, there were lots of programmers :)