Anyone who was previously playing in the "Friends and Family" beta automatically has access to the closed beta, and current Diablo 3 players can copy their existing characters to the beta once every 72 hours.
The Reaper of Souls beta, which is available on both Windows PC and Mac, contains all the content in the expansion, including the final boss fight. Blizzard is using the beta to gather feedback so it can make gameplay tweaks and fix bugs. The studio is maintaining three separate feedback forums on Battle.net: feature and system changes, class changes and other items. Forums for bug reporting and technical support are also available.
"As this is a test server, please anticipate uneven game performance, and note that restarts and downtime may occur without warning," reads a note from Blizzard in the beta FAQ. "We'll provide information regarding extended downtimes, should they occur, in the Reaper of Souls General Feedback forum."
Reaper of Souls will be released in 2014. For more details, check out our coverage of the expansion from BlizzCon 2013.