There's going to be limited slots so it will either be a first-come, first-served basis or randomly-chosen.
However, if you really want to play the beta, pre-ordering it will guarantee you a spot. Check with your local retailer if they are giving out codes for pre-orders!
XBox One players will get early-access to the beta this december, while PC and PS4 players will get a try in early 2016.
Check out the full FAQ here.
Good to see more games opening their doors to betas. It's a great way to test out the servers, bugs & game balance among others. Ubisoft did it last year with The Crew and I believe it was a smooth release when the game lanuched. Evolve also did several testings and they too had a smooth release.
I'm still undecided on The Division so hopefully I get a chance on the beta and make my decision from there.