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SA and VT (UF is still inactive) will let in people as they choose. The rules for recruiting are on the SA header in the SA thread, and I believe the same is true for VT. So, basically, you ask someone in SA or VT if you can join, that person brings you up to the leader/officers/ w/e, they may bring up your name to everyone in the army, and over the course of a week or two, a decison is made. Vet status is not a qualification (skill is considered though).
Note: I'm not 100% accurate on VT, since I've never been in VT. I think that's about the way they work though.
EDIT: Stiff was let into SA because the members of SA approved of him. Nothing more, nothing less.
"Life before death. Strength before weakness. Journey before destination." - First Oath of the Knights Radiant (Stormlight Archive)