In order to join an existing championship, you will need an invitation code that your friend who created the championship sent you.
Either follow the link a friend sent you, or if you have the invitation code, simply go to the Events screen, access Friendly Championships and choose the “JOIN” option. A pop-up will appear where you can paste the invitation code to view the Championship.

Once the Championship screen opens, you can request to join the Championship on the button placed in the upper part of the screen.