If you want to get into the beta go to http://www.curse.com/keys/age-of-empires-online/default.aspx. You'll need to sign up for a free account. I just set up a junk email address and used that. Beta keys seem to appear randomly throughout the day so keep checking back. What I did was install a Firefox addon that reloads the page every minute so I got mine after about an hour.
Also keep in mind that this is a Microsoft game. Yeah, that's right, you'll have to dike around with GFWL to download the game files. I ran into all sorts of problems getting it to accept my user account and beta key but eventually it went through. I actually had to set up a new live account and it gave me this stupid default user ID that I haven't got around to changing yet. Ah, the things we go through for free stuff.
I don't really know too much about it other than what I've surmised from previous installments in the series and screenshots. If anyone else has played it feel free to chime in on your experience.