This is pretty cool. After clicking on a link in the Google invitation, an application is downloaded to your PC. After installing it, you simply identify where your music library is on the PC ie. Windows Media Player, music folder or whatever, and the program uploads your entire library. It takes a while and my upload is still in progress as I write this.
I had the Music app preinstalled on my A500. When you open the app, it askes you to sign in to your Google account, and it immediately starts to sync all of uploaded music to the tablet.
Still fooling around with it but the interface is very slick and polished in the way it displays the music and the options you have to filter the music by artist, genre, etc.
Will post more after I play around with it some more, but I have to say this is impressive.