To join MWM, you actually join American Mensa, and tell them that we are your "chapter of preference".
So how do you join American Mensa? You can qualify by sending in your scores from any one of many tests, listed on the American Mensa web site. (Note that they'll review it for free in the month of December 2015 -- normally a $40 fee!) Or you can take the Mensa exam, given at one of our MWM Testing Sessions.