It's really not very difficult to get into MENSA. The test is about the equivalent of a US public school gifted program. I think the "average" hackernews user would pass without much of a problem. It's far more difficult (and prestigious in my opinion) to get into YCombinator.

If you're looking to meet interesting and intelligent people, you're probably better off finding a community oriented website with content that appeals to you and going to meet ups. Personally I would much prefer an HN meetup to a MENSA gathering.