Oh, It's all volunteer. Anyone and any brand bike is welcome. Although most of the riders are vets you don't need to have any military association at all.
To join, you MUST register the night before or morning of the first day that you will be riding. You only register once. You are given a wristband that entitles you to gas/food. Without a wristband they will ask you to stay away from the group until you do register. Riders meetings each morning are mandatory before you can ride in the formation.
The purpose of these rules isn't to be arogant or rude. RFTW rides side by side most of the time with 2 second intervals between rows. If someone jumps in along the way and doesn't understand whats going on they can get a lot of people hurt. They cover all this each morning at the riders meeting.