I almost never play music but I went ahead and requested an invite for the beta.
Looks like you download and install their music manager and show it where your
music is and it uploads it. Install it on another machine or device and it can play
all the songs. Saves the last songs you played so they'll play offline. Seems like a
good idea. Of course you will at some point be able to buy songs from the Google
music store.
You can request an invite
HERE