I guess this thread is the...unofficial...official response (Reddit AMA)?
"BUT here's the good part. Stadia IS NOT A BOX! You don't need our hardware to start playing, remember? Right after we ship your order (but not earlier than 9AM PST 11/19), we'll send you an email with the invite code. You can use it immediately to create your account and reserve your Stadia name in the Stadia app on Android or iOS. Just for clarity: the first day is a little special, if we ship your order on 11/18, you’ll receive the code on 11/19."