This is a critical moment in the game development. If the team does not choose correctly the alpha testers, they will have lots of people complaining about the game after it is released, unless the team is perfect, managing to avoid all kinds of bugs and thinking all of the critical features the game should have by themselves. Look at games such as BF4, which is so buggy (specially netcode and weapon balancing) because of lack of testing before the game launch. Because no human is perfect (even the awesome developers of US2), alpha testers should strive for helping the team, so they can build a great game.