hi, the plugin does not support this kind of feature. With out-of-the-box functionality an admin will always need to create the leagues and also put people in it. You can of course ask in the invite to select the correct league upon registration. You can create ranking pages per league with the shortcode. And if you use a membership plugin or something that supports directing users to pages based on profile, you could also limit the pages that users can see (so they only see the relevant ranking for their league).
If you have PHP knowledge, you could write your own extension to the plugin that hooks into the league creation and display of rankings. It should be possible to write something that allows users to create their own league and to create invitation links for that league. But it won’t be a simple extension (meaning: your would need to put quite some effort in it).