The default behavior for routing in Phalcon is: /{controller}/{action}/{parameters}
For the example you want, you should have a GroupControll.php with an indexAction that receives the "6" as the action parameter.
To do this, without having to set the request as "/group/index/6" to respect the default route, you have to create a new route so that you can keep the uri the way you want.
You can create a new route like this:
public/index.php
    //Set routes
    $di->set('router', function(){
        $router = new \Phalcon\Mvc\Router();
        $router->add(
                '/{controller}/{id:\d+}',
            array(
                "action"         => "index",
            )
        );
        return $router;
    });
and in you GroupController.php:
class GroupController extends \Phalcon\Mvc\Controller
{
    public function indexAction($id=null){
       $this->view->disable();
       echo "Group Controller; IndexAction; ID $id";
    }
}