https://www.gamespot.com/articles/sea-of-thieves-beta-now-available-but-some-players/1100-6456306/

The Sea of Thieves closed beta has officially begun. The beta is going on now for select Xbox One and PC players, but it appears many are currently having issues accessing it.

Like the Sea of Thieves technical alpha that took place back in December, the closed beta is only available to those who either joined the Sea of Thieves Insider Program prior to December 1, 2017 or pre-ordered the game. Players who fell into the latter camp received the beta automatically if they pre-ordered Sea of Thieves digitally, while those who reserved the title at retail had to redeem a code in order to install the beta. However, it appears many pre-purchasers are being met with a "too early" error message when they attempt to launch the beta.