Unfortunatelly, for IIS installed on Windows 7/8 machines, there is no option to create users only for IIS authentification. For Windows Server there is that option where you can add users from IIS Manager UI. These users have roles only on IIS, but not for the rest of the system. In this article it shows how you add users, but it is incorrect stating that is also appliable to standard OS, it only applies to server versions.