Public testing on the expansion is already underway with Blizzard's "Friends and Family Beta," which includes friends and relatives of Blizzard employees as well as "valued media contacts and key members of the Blizzard community." The beta testing phase will expand by the end of 2013.
"Our Friends and Family Beta is just the first step of public testing, and by the end of the year we will be moving into Closed Beta," the post reads. "Since our Reaper of Souls Closed Beta test will also feature all content in the expansion (again, excluding the final boss fight), invitations will be extremely limited. However, we'll continue handpicking community members to invite to the Closed Beta and bringing in additional players through opt-ins to ensure that all of our features like the Crusader, Mystic, and Adventure Mode get the testing they deserve."
Those interested in participating in the closed beta need to adjust their Beta Profile Settings in their Battle.net profile.
You can learn more about the Diablo 3: Reaper of Souls by reading Polygon's coverage from BlizzCon 2013.