The requirements for joining Mensa can be found at Mensa.org. The primary requirement is being a good test taker. Meeting this requirement generally means having a higher level of intelligence than your bad test takers. However, having a high IQ doesn't indicate any other characteristics about an individual. I had been somewhat of a disestablishmentarian during what should have been by college years and ended up with a lot of fond memories, but no degree and no test scores that met requirements. I had heard about the idea Mensa was founded on, and decided to try it out. I took one of their entrance tests and passed. After being a member for just 4 months now, I have observed several things.
Having intelligence does not mean you will use it. Or that you have a desire for wealth, fame, or power. It seems Mensa is full of mostly ordinary people from all walks of life. The difference is the level of conversation you can have. At gatherings, I find it much easier to find someone to talk to on just about any subject. And I know that if the subject changes to pop culture, the WWF, or even the weather, it's not what you would expect from most people, or is it what you would expect from arrogant know-it-alls. And it's a pleasant change for me.
At a gathering, I might talk with a retired physicist about field theory, and then turn around and talk with someone 30 years old, still living in their parent's basement, but who never the less has some interesting ideas about encryption technology. And then I might talk to a "career housewife" about the condition of public school education and discuss the advantages and pitfalls of home schooling. It all depends on who shows up.
Which brings me to my next point, Mensa, like most other voluntary organizations, operates with about the same percentage of activity. 30% are 'active members' and go to gatherings on occasion. 20% are regulars. And 10% are active enough to participate in the governing and administrative duties of the organization. This varies from org to org and when the big events come around (like Easter for Christians) the church is sometimes quite full. But there are still lots of people in Mensa that sit around waiting for things to happen not understanding that they have to make them happen. Just like in real life. :)
On the whole, Mensa is full of people that can appreciate higher level concepts. They wont be as influencable by mass marketing techniques, they generally don't position our societies precious "celebrities" on as high of a pedestal as most icon worshipers do, and they usually prefer a good discussion to a donnybrook. These are just tendencies that any group of intelligent people would have, and each member has to a greater or less degree. There are exceptions, and I do know of at least one "out of the closet" racist in the local organization I am a member of. So obviously with intelligence does not necessarly come morals or common sense or whatever ingredient is lacking in racists. But any good Mensan would be glad to discuss with you statistics on the matter.