Set Drupal group parameter on a views page route with a group ID contextual filter.

MyModuleRouteSubscriber.php

<?php

namespace Drupal\my_module\EventSubscriber;

use Drupal\Core\Routing\RouteSubscriberBase;
use Drupal\Core\Routing\RoutingEvents;
use Symfony\Component\Routing\RouteCollection;

/**
 * My module route subscriber.
 */
class MyModuleRouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection->get('view.my_view.page_1')) {
      $parameters = $route->getOption('parameters');
      if (!$parameters) {
        $parameters = [];
      }
      $parameters['group']['type'] = 'entity:group';
      $parameters['group']['converter'] = 'paramconverter.entity';
      $route->setOption('parameters', $parameters);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events = parent::getSubscribedEvents();

    // Use a lower priority than \Drupal\views\EventSubscriber\RouteSubscriber
    // to ensure the requirement will be added to its routes.
    $events[RoutingEvents::ALTER] = ['onAlterRoutes', -300];

    return $events;
  }

}