register/create a new middleware? and then check if user_id == 1? check other middlewares for examples.

I guess my first question is why are you trying to allow a certain user the permission to register another user while they are already authenticated? Why don't you just create a controller and a view that allows a form submission for a new user and then provide access to that user to use that controller?

I say this because currently by default there is middleware setup to redirect if authenticated. Please see app\Http\Middleware\RedirectIfAuthenticated.php.

Warning: Below is not well tested and likely not the best way of doing it, but here goes.

In Auth\AuthController.php add:

    public function getRegister()
    {
        if (!Auth::check() || Auth::user()->id !== 1)
            return redirect('/');
        return view('auth.register');
    }

This will override the default controller method and redirect people accessing /auth/register if they're are not logged and/or if they don't have user id 1.

Next you will probably have to add getRegister to except in the AuthController constructor, L5 by default redirects auth'ed users away from auth/login and auth/register, so try the change below.

    public function __construct()
    {
        $this->middleware('guest', ['except' => ['getLogout', 'getRegister']]);
    }

Alternative, you can change the RedirectIfAuthenticated.php middleware, but I think above is a nicer way of doing it.