David_Jay's explanation is correct. Score is not the qualification; it is percentile.
My grandmother and mother were/are members so I joined after exiting. Chapters vary as they are made of people who vary. While I've done very little with the group (scored some scholarship essays, etc.), I enjoy the publications.
It has never been my experience that Mensans think they're "special" or whatever. It can be difficult to find others with like abilities at times so....the round table makes sense.
Since I have so little to do with the group (other than reading the publications), I often found it difficult to justify the dues so I became a life member.
There are some obviously bright folks on this board and elsewhere. Mensans certainly don't have the "intelligent interaction" market cornered.