"Geniuses" are defined by what they do, not by a label conferred on them.
We know Einstein, Newton, and Sagan by what they did, not by a label by some organization. If it wasn't for what they did, no one would ever consider them a genius -- and rightly so.
If you want to be anything in your life, find a place to make a difference.
Mensa is not a place where you can make a difference. It's an exclusive club defined by their own standards, nothing more.
A club does not a genius make.
You want to be a genius? Then DO as a genius DOES. That means making a contribution of knowledge in some area. (And guess what? No one needs to be a genius to make a contribution to society, science, history, or anything else. They just have to have a vision and work hard, and be one of the lucky ones that accomplishes something before they die (that's harder than you think).)
It's that simple. And you do not need an organization for that. It's not the dusty, stuffy halls of Mensa that makes a difference and contribution to society. It's the research labs, the soup kitchens, the people working hard in every area of life ... and whether or not they are geniuses is irrelevant.
Do you want to make a difference? Then ACT to make a difference.