If you've opted in for the beta, and are eager to try out the game's new Crusader class, make sure to check your email in-boxes for the official invitation. For those still interested in joining, step-by-step instructions can be obtained here.
For participants, Blizzard is encouraging players to livestream, record feedback, and take as many screenshots as their heart's desire -- as there is no non-disclosure agreement tied to the closed beta. The full content of the expansion -- excluding the final boss fight and auction house -- is also included.
The Diablo III: Reaper of Souls closed beta is open to all Windows and Mac users, and is available only in English and Korean for the time being. Additionally, there is currently no date to when the beta will end.
Reaper of Souls™ Closed Beta Now Available [Battle.net]