Using windows authentication for mvc app, where to start???

I am writing a web application to replace the one we are currently using. I have finished most of the application and now need to integrate the authorization and administration into the mvc views and controllers. I understand how to use the "authorize" attribute on controllers to hide views and use "is authorized" in views to show different content based on roles and users. What I need help with is the authorization setup and in app management. The app we currently use is set up so that admin can enter the "domain\username" of an employee and assign them an authorization level (I assume a custom role). This allows us to issue assignments to the users so that they only see their assignments. I am unsure if this done through windows authentication in an intranet application, or forms authentication in a web application. If its not windows authentication and custom roles, how do I tie-in the domain accounts to a forms authorizat ...