For players that have yet to receive an invitation to the closed beta, make sure you've opted in on your Battle.net Beta Profile Settings page -- and while you're at it, make sure the page has the latest system specifications for your computer. Keep in mind that with the closed beta, you must have an active Diablo III license attached to your account -- and that license needs to be in good standing in order to be eligible. No banned or suspended accounts.
Also, as with any beta test, the risk of phishing attempts runs a little higher. If you receive an email that states you're in the beta, be sure you don't click any links in the email itself. Instead, log into your Battle.net account to check that the license has been added, or to manually add a game code. For more information on the Diablo III: Reaper of Souls closed beta, take a look at Blizzard's official page -- and good luck to all those that have opted in!