To compete in a Multiplayer Champions Cup you need to register for that cup in advance.
Each cup has a registration period before the cup commences.
You will receive a notification and your assistant will let you know when registration is open.
During the registration period, you can also cancel your registration from the cup's registration screen (Edit Lineup screen) 'Menu' button. Tap on the cup menu panel to access the Edit Lineup screen.
* You can modify your Lineup any time during a cup to play matches but it will not affect other users.