Bug in routes with path in controller

Am also experiencing the same issue with routes with parameters

PHP Code:

    $routes->group('usergroups', ['filter' => 'ajax'], function($routes){
        $routes->add('(:num)/edit', 'Backend/Users::editGroup/$1', ['as' => 'edit_group']);
    }); 


This route is not working. Am getting 404 error. Controller method is not found: index

I have controller in:  App/Controllers/Backend/Users.php

PHP Code:

<?php 
namespace App\Controllers\Backend;

use App\Controllers\Backend;
use App\Models\Backend\UserModel;

class Users extends Backend
{
    protected $userModel;

    public function __construct(){
    parent::__construct();
    $this->userModel = new UserModel();
    }

    public function editGroup($groupId)    
    {
    $data['group'] = $this->userModel->getGroupDetails($groupId);
    echo view( 'backend/users/edit_group', $data);
    }
} 


how can i fix this issue?