I've seen Mensa and other organizations online - here's a list.
http://www.eskimo.com/~miyaguch/history.html
What's the point of joining them, though? If it's just sitting around feeling superior to others, then I really don't want to be a part of it. Intelligence also seems quite complex, and standardized tests would capture a few facets, but not the entire picture. For example, the general GRE went well for me, but there are a number of other areas where I'm quite weak. How do you measure the big picture?