From the official forums... https://forums.ageofempires.com/t/upcoming-age-iii-de-beta-sessions-read-all-about-them/67530
Hello and Happy New Year, Age Insiders! We thought we'd start this year off by sharing some exciting news with you all first. Starting early next month, we'll begin inviting select Insiders to beta various elements of Age of Empires III: Definitive Edition 407!
If you joined us for the Age II: DE closed beta last year, we'll be structuring things a bit differently this time around. Instead of one longer closed beta with a gradually expanding audience, we'll be splitting the Age III: DE beta into several different, shorter sessions, which will feature focused slices of the game.
Take some time before Jan. 28 to polish up your Insider settings via your Profile and let us know what and where you like to play! And remember: whether you're invited to an Age III: DE beta session or not, there will be loads of other ways to take part as an Insider.