Update column and table structure advice

[lukep11a]

I have a user interface where a user can create a new private league, the form submits a private league name, a passcode and automatically generates an ID field for the private league. What I need to do now is create some way of listing who has joined each private league, do I add a private league column to the user table and update the row of each user when they join a private league? Do I have extra columns on the private league table to specify each new member that joins? But I do not know how many people are going to join each league and don't want to limit the number by inserting a specific number of columns. I am really struggling with how to best structure this so am just looking for abit of advice really. Any help would be really appreciated.