Starting on November 9th, November 30th, and December 7th, the betas will run for 48 hours. During each of those weekends, there'll be a specific theme to the testing. You'll have a chance to call down thumpers like a lunatic, wage war against indigenous wildlife, and beat back the Chosen.
Get more information or sign up on the beta page on the official site. Note that not everyone who signs up is guaranteed access; the team will be sending out invites to registered players in batches throughout the beta weekends.
[Update: We've clarified that not everyone who signs up is guaranteed a spot in the beta, though with the planned stress tests, your chances look pretty good!]