When Google enables it, you'll be able to join a waitlist for an out of stock item from the listing page. The page can also tell you if the waitlist is full and provide an option to leave the waitlist. When joining the waitlist, you'll see text reading, "You've joined the product waitlist. We'll send you an email at [email protected] when it's available."
This doesn't appear to work with products that are not yet released, just those that are out of stock. We expect Google to flip the switch on the waitlist features any time now.
It looks like the waitlist features have been live in some countries for a few days, but we only just spotted the changes on the US site today. It might still be rolling out.