Described as a “limited, closed public beta” – the only one prior to launch – you must receive an invite to participate. Eligible people must have signed up on their beta page previously, but “not everyone who signed up will get an invite.”
The invites are being sent out right now, with the beta running from Thursday, April 23 at 7am PT/10am ET through Monday, April 27 at 7am PT/10am ET. There is no NDA in place, and they encourage you to share screenshots and livestream your playtime.
For a few other details, Bethesda confirmed all characters from the console beta will be wiped before the console launch, meaning the beta characters won’t transfer over. Noting that this isn’t a cross-console beta test, there will be two megaservers for each console – one for North America and one for Europe – and you may choose which one to play on, but characters are unique per megaserver.
One big factor with The Elder Scrolls Online beta is that it’s the full version of the game. Because of this, the main download will be 50GB, with a 15GB day one patch.
The Elder Scrolls Online: Tamriel Unlimited will be available on June 9 for PS4 and Xbox One.
[Source: Elder Scrolls Online]