Craft Commerce: Add user to a group when they make a purchase

MyModule.php

<?php

namespace modules\mymodule;

use Craft;
use craft\elements\User;
use craft\helpers\ArrayHelper;

use yii\base\Event;

use craft\commerce\elements\Order;

class MyModule extends Module
{
    /**
     * @var int ID of the special group.
     */
    const MAGIC_GROUP_ID = 123;

    /**
     * @var int Purchasable ID that triggers adding someone to the special group.
     */
    const MAGIC_PURCHASABLE_ID = 456;

    public function init()
    {
        parent::init();

        Event::on(
            Order::class,
            Order::EVENT_AFTER_COMPLETE_ORDER,
            function (Event $e) {
                $order = $e->sender;
                $lineItems = $order->getLineItems();
                $user = $order->getUser();

                // Were they a guest?
                if (!$user) {
                    return;
                }

                // Nothing to do if the order doesn't contain our magic purchasable!
                if (!in_array(self::MAGIC_PURCHASABLE_ID, ArrayHelper::getColumn($lineItems, 'purchasableId'))) {
                    return;
                }
                
                // (The above could be a more sophisticated check for, say, whether any products are related to a specific category… or even to grab a `Group ID` from a field on the purchased product!)

                // Fetch just the existing Group IDs:
                $groupIds = ArrayHelper::getColumn($user->getGroups(), 'id');

                // Push in the new one:
                $groupIds[] = self::MAGIC_GROUP_ID;

                // Assign them back to the user, committing to the database:
                Craft::$app->getUsers()->assignUserToGroup($user->id, $groupIds);
            });
    }
}