With this snippet you can create a page template restricted only to those users with the right role to access to it.
You will need to create the include, in this case error.php – this include should contain the message you wish to show. If it is not the user it is intended for your error.php might say: “Sorry my friend, the force is not strong enough with you!”.
If you create custom roles you can do the same ie. buyers, clients etc.