There are two types of league: private leagues and public leagues.
You can create private leagues for you and your friends by going to the ‘Leagues’ section and selecting ‘Create a league’. From there, you can invite friends by sharing the league link or league code.
If you’re trying to join a league that a friend has set up, you’ll need the league link or league code. Ask the league admin to share it with you.
Every player is automatically included in the overall leaderboard. On top of that, if you’ve selected your country and favourite club in your account settings, you’ll also be included in a league with other players from your country, and all other people who have selected the same favourite club as you.