Diablo III's closed beta test for the Reaper of Souls expansion will begin by the end of the year, Blizzard said today. It's underway now for "friends and family," of Blizzard—which includes "3,000 of our most active Diablo III players from around the world" in addition to VIPs and press.
When the Closed Beta begins officially, players will see everything they do (except the final boss fight, which no one gets to play.) However, "invitations will be extremely limited."
If you're looking to get into that beta, you need to opt-in for beta testing. You do that by going to the Account Management setting of your Battle,net Account and adjust the beta profile settings to show that you're opted-in for Diablo. It doesn't guarantee access, but it gives you a chance to get in.
Friends and Family Beta Test Underway [Battle.net]