You could instead have “users” that are the teams themselves, regardless of whether there are also player users on the team or not. If the team “users” are assigned a specific role, it’s trivial to get the wp_dropdown_users() function to only list “users” of that particular role.

You would generally relate WP users to other data by their respective User IDs. This is how the usermeta table is joined to the users table when writing SQL queries. Similarly how authors are related to their posts.