After you have created an event, join it by tapping on the event card. You will see an “Add camera” button. Set the camera up with the viewfinder placed at the angle you want to capture. That's it–That device is now filming for everyone at the game.
You do not have to press anything to begin recording. Once the viewfinder opens, the camera is running. Although it may seem as though the camera is open but isn't saving anything, once you tap the button, you will be able to replay the past 15 seconds from that camera's viewpoint.