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? There are several ways (see https://www.us.mensa.org/join/ for more details), but the main two are:
Send in your scores from any one of many tests, listed on the American Mensa web site, that you may have taken in the past. We call this "Prior Evidence Review". Normally there is a charge of $60, but during March 2019, this fee will be waived!
Take the Mensa exam, given at one of our MWM Testing Sessions (or testing sessions run by some other chapter).