As soon as you have configured the roles API, by following approach described in the article How to use Roles API for Role-Based Authorization in ASP.NET you can create users and roles and then assign users to these roles using either the WAT or the Roles class in your code. You should click the Create or Manage Roles link on the Security tab. Then you can create roles and add users to roles, as shown in the next picture.
Adding users to roles