Exactly what Geo said, there is a core list of people that will be givin an invite to the se5 beta, but this in no way guarantees that they will be available (as in 'have the necessary time to spend hunting bugs') The random selection does two things, it indeed gives a chance for new gamers to get into the beta but it also brings new blood into the mix. Often in the beta process a newbie will find things a veteran will miss, simply because the veteran knows how to play and is less likely to make wierd or 'different' choices when going through the different parts of the game.
All in all, there is allot to scan and test during beta. It always starts out as a massive bug hunt, and then slowly it becomes more stable and the beta testers get to make 'suggestion' for gameplay imporvements. At the end of the beta all the testers can be proud of a job well done, but you should know that a beta does not end at the release of the game, beta testing still goes on after the release of the game. (just that everyone is now a beta tester
)
Anyway, the countdown has begun, so stay tuned.
Cheers!
Edit: Fixed embarrasing typo