Is it possible? Yes, but you’ll need to do some coding. There is a filter that can be used to limit the users that are shown. This example uses the filter to remove inactive players, but you can also change it to filter out users based on a selected league ID.
Then all you need is a dropdown. Every page in the plugin also has a filter to add content. A (somewhat changed) example from the help page (untested):
add_filter( 'footballpool_pages_html', 'add_dropdown', null, 2 );
function add_dropdown( $content, $id ) {
$dropdown = '';
if ( $id === Football_Pool_Utils::get_fp_option( 'page_id_statistics', 0, 'int' ) ) {
$dropdown = '<select name="league">
<option value="1">test 1</option>
<option value="2">test 2</option>
</select><br>';
}
return "{$dropdown}{$content}";
}
You can make it a bit more complex by retrieving the leagues dynamically, but if they won’t change, then that might be overkill.
Another option would be to just add 3 links to the same page but with the league ID in the querystring, and then use that value from the URL in your code to filter out the users.
p.s. I noticed on your website that your teams are not all labelled as ‘real’ teams. You can see this, when you go to the groups page.