I'm a very active BC2 player; they way they choose who gets an invite, is not only by your online activity with BF series, but also game order amount, time with an EA account, and whether you are subscribed to their newsletter.
Some factors matter more than the others, but that's pretty much it. If any of you guys meet all or most of these criteria, and haven't received an invite, it will then just be a matter of time.
It actually does, though you don't sign anything, I got invited too (YAY!) in the email they send, they tell you to refrain from posting pictures, gameplay videos, or personal impressions of the game.
dont think you can sell or give keys, because you generate it through your own account when they send the email, you'd have to give them your EA account