On the game page, tap Get notified when a spot opens. It's free.
When someone drops out, everyone waiting gets a Someone dropped out! notification at the same moment. The spot goes to whoever books first, so book as soon as you see it. Being on the list doesn't hold a spot for you.
You're notified once per game, so make sure notifications for GoodRec are turned on in your phone's settings. See How do I manage my account and notifications?