According to the Blizzard website, Diablo III: Reaper of Souls is now accepting opt-ins for their closed beta test.
Diablo III Beta Now Accepting Opt-Ins, Will Not Perform Key Giveaways
Invitations to the next phase of the Diablo III: Reaper of Souls public test are now being delivered to opt-ins, worldwide. Keep in mind that Reaper of Souls is still in development, and content therein isn’t necessarily representative of the final game. Blizzard also made very clear that they would not be using Social Media or any other method to giveaway closed beta keys. *cough* Hearthstone *cough*
For additional information about the Closed Beta including how to participate visit Blizzard’s beta site here.