Audience Manager is a subscription management tool which will allow visitors to manage their email subscriptions with your organization. I've seen many clients abuse this to double as an identity management solution but it is not intended as one so I would not advise this as a stable, production grade solution.
You can use .Net built-in authentication and authorization providers (for example; you could set the authentication mode of the web application to "Windows" and login with ADS users, get group memberships of these users through the ActiveDirectoryMembershipProvider) to achieve this (I'm not well-versed enough in Java to know how this is normally done there).
SDL Tridion Content Delivery itself does not act as an authentication or authorization solution for two reasons:
- these concepts are implemented in totally different ways in Java and .Net meaning there is no single codebase possible to solve both
- SDL Tridion is an enterprise CMS and as such their customer base will expect the solution to integrate with whatever authentication and authorization solution they are currently using making a tight integration with a single solution counterproductive for implementation partners
Maybe a "generalized" implementation in the Tridion Reference Implementation is possible/foreseen/already there however TRI is not formally part of the product but a community driven initiative.