Hi, I'm a moderator of the league. How can I change the settings (without spending boss coins) that any player can join the league?
You don't spend coins for changing your league to private or public.
However, being able to change your league from private to public depends on if you used any advanced tools when creating the league.
For the website, you can find the moderator tools on the dashboard page while for the app you can find the moderator tools on the menu list.