AU,
Whether or not you join is a personal decision. From my experience, I have two family members who were part of Mensa. It is an organization that you can make of what you will, and take from, what you will.
Personally, my experience has been that IQ is much less important than common sense. I have seen many of high IQ, not be able to get around in this world.
I can also, honestly say, that, of the folks I have met that have extremely high IQ's, that, quite often, they are actually not as happy nor as well-adjusted, as some others. My nephew, 43 now, been in Mensa forever, graduated high school at 13, finished college with 2 degrees by the time he was 18, has the social awkwardness of a 12 year old boy. But, 5 will get you 10 that, if you play video games - his hand has been in the works.
(he is a consultant for several of the major computer game companies).
But, as for getting along in the world, no friends, no relationships, no hobbies, nothing. He finds it very difficult to carry on a normal conversation.
And, he is not alone.
Sometimes, we push people too far with their intelligence. I firmly believe that, pushing his schooling beyond his physical years, left a hole in his growing up, the kind most folks get by interacting with people (kids) their own age.
So, its not a case of your smarts, as much as it is what you do with your life. (just my opinion).
Beth