It is currently the ONLY group that I am an active member of. I will probably soon become a life member, and I doubt that I will ever join another group. It is the only group that has meetings all over the US, and they have local groups in many other countries as well. I have made several good friends through Mensa, and I recommend it highly as a social outlet for bright people from all walks of life. And no, it's not just a bunch of nerds sitting around talking about Star Trek and mathematics, although we do that stuff too.
Children are welcome at most meetings, but some are specifically for adults.
There are many places where you can take the tests, in some cases by appointment, or on scheduled group test dates. Camelopard Society - This group has been defunct for some time now. They were active in California for over 10 years as a group that seceded from Mensa. They had and I think still do have many meetings in their local area, but no longer publish a newsletter. I was a long distance member, and can say that their newsletter was high quality, along with many of the group's members - many very nice people.