Team.php

<?php

use SilverStripe\ORM\DataObject;
use SilverStripe\Forms\FieldList;
use SilverStripe\Forms\TextField;
use SilverStripe\Admin\ModelAdmin;
use SilverStripe\Forms\GridField\GridField;
use SilverStripe\Forms\GridField\GridFieldDetailForm;
use SilverStripe\Forms\GridField\GridFieldConfig_RelationEditor;

class Team extends DataObject
{
    private static $db = [
        "Name" => "Varchar(255)",
    ];

    private static $many_many = [
        "Supporters" => [
            'through' => TeamSupporter::class,
            'from' => 'Team',
            'to' => 'Supporter',
        ]
    ];
}

class Supporter extends DataObject
{
    private static $db = [
        "Name" => "Varchar(255)"
    ];
    private static $belongs_many_many = [
        'Supports' => Team::class,
    ];

    public function getCMSFields()
    {
        $fields = parent::getCMSFields();

        if ($this->ID) {
            $manymanyfields = FieldList::create(
                TextField::create("Name"),
                TextField::create("ManyMany[Ranking]", "Ranking")
            );

            $config = GridFieldConfig_RelationEditor::create();
            $config->getComponentByType(GridFieldDetailForm::class)->setFields($manymanyfields);

            $grid = GridField::create(
                "Supports",
                "Supports",
                $this->Supports(),
                $config
            );
            $fields->findOrMakeTab("Root.Supports")->replaceField("Supports", $grid);
        }

        return $fields;
    }
}
class TeamSupporter extends DataObject
{
    private static $db = [
        'Ranking' => 'Int',
    ];

    private static $has_one = [
        'Team' => Team::class,
        'Supporter' => Supporter::class,
    ];

    private static $default_sort = '"TeamSupporter"."Ranking" ASC';
}

class TeamAdmin extends ModelAdmin
{
    private static $managed_models = [
        Team::class,
    ];
    private static $url_segment = "teams";
}