How are people playing betas before they come out?
Beta tests are accessible in different ways, depending on how the developers want to do it.
Ways to participate in private beta:
- Be a part of the developer team
- Know someone on the developer team
- Know someone who knows someone on the developer team
Ways to participate in public beta:
- Complete a sign-up process before the beta begins, then be randomly selected
- Buy a game which contains beta access to another game
- Pre-order the full game
Essentially, you just have to know how that particular beta is being carried out, and/or be very lucky.
The developers can be playing a "beta" before it's even a beta...in which case it's an "alpha". In the case of Gears 3, Devs played it first, then they allowed friends and family, then they allowed Bulletstorm purchasers, and finally they allowed pre-orderers.
Your hidden question is not "How are people playing betas before they come out?" but "How can anybody play a beta before it comes out FOR ME?" In this case it's actually the case of "you gotta know somebody".