If you receive a notification that you have an invite waiting, you can open it up and either Accept or Decline it. If you Decline it accidentally, the other coach will have to send it again.
Once you accept the invitation, a new Cross-Account game will appear on your Games page. You will automatically be sent to the page where you can map your trackable to the trackables set by the original coach. You can find info on how to do that in this article. If you don’t want to map trackables yet, that’s totally fine. You can go back to it any time, just remember that your scores won’t show up on the game until you map the trackables!
As an invited coach, you won’t be able to edit schedules or add new trackables, but you can always add or delete your own players from a game, or add players from any other agency if you want to issue challenges of your own and amp up the competition!