They will be primarily selecting based on your computer rig, as said before.
The biggest reason for large tests (other than server stability etc) is to check for issues with different rigs.
By the time a game gets to public beta, its usually not really about the bugs anymore. Although finding bugs is ofc great, its more about the above, and seeing if people find glaring balance issues/breakable content etc.
As for inviting pvp guilds etc, they already did invite pvp guilds to some of their previous events and the likes. I'm sure they will again, but its likely they will stick to guilds they know from gw1/top guilds from other games with a good reputation etc if they want directed feedback from a whole guild.
Just an fyi, there are whole companies dedicated to bug testing in beta's etc. Hence why its usually not really the main objective of the public betas. (Feedback from public betas can often be more than a little overwhelming, and most people dont know how to properly report bugs and reproduce them etc)
This post was edited by silentdeathz on Feb 28 2012 01:11pm