Easy, check out the Adoption Center and look at the Pokemon each squadron has available to it. See which one your favorite Pokemon are listed under and Email TRCassidy with what sqadron(We divided Pokemon into a bunch of categories which we call squadrons) you'd like to join, what you'd like your team to be(not nessasary, more of a fun thing. Maximum of 6 Pokemon per team) and what your screen name will be(basically, choose a name we can put on the member page).  That's it ^-^.

Q. Does it matter what squadron I join?

Nope, each squadron is the same pretty much, except some have Pokemon available that the others don't. So basically, if you like Flying Types, join Crobatty's Airforce. If you like water types, join Bullados's Navy. Ditto for the other squadrons.