Invitations are being delivered to nephalem around the world, as fans return to Sanctuary to check out exactly what's new and different in this new expansion. Already in the Friends & Family beta? Carry on, your access is unaffected.
If you've been invited, Blizzard also wants to make sure you know there's no restrictions on what you can talk about:
To make sure you have the best chance of getting a Beta invitation, you need to opt-in for beta testing. Head to your Battle.net account, visit your Beta Profile Settings Page and make sure that "Diablo" is ticked under the "Beta Tests I'm Interested In" heading. Update your preferences, and you're good to go (well, you'll need an active Diablo III account in good standing, but you've got that already, right?).
Once you're in, it's your job to give feedback and bug reports to the developers. There's a number of ways you can do this, primarily via the Diablo III forums - and you'll need a Closed Beta license to access the threads.
Remember, this is a beta in its truest sense - you're testing an unfinished and potentially uneven game, and restarts and downtime "may appear without warning". If any extended downtimes are expected, you'll be told about them via the Reaper of Souls General Feedback Forum.
There's no release date for Reaper of Souls just yet, but it's expected to be some time in 2014.