If you would like to join a game, clicking the Register button will add your name to the list of players.
If you're already on the list, and would like to remove your name from the list, press the Unregister button to cancel out.
If a game is already full, press the Join Waiting List button to add yourself to the waiting list. If someone who is scheduled to play decides to cancel out, your name will automatically be added to the list of scheduled players. (You will be notified if/when this happens).
There is no limit to the number of players joining the waiting list. When a seat opens up, players are moved from the waiting list on a first-come, first-serve basis.