ACS supports the authentication of computers that are running the Microsoft Windows operating systems that support EAP computer authentication. Machine authentication, also called computer authentication, allows networks services only for computers known to Active Directory. This feature is especially useful for wireless networks, where unauthorized users outside the physical premises of your workplace can access your wireless access points.
When machine authentication is enabled, there are three different types of authentications. When starting a computer, the authentications occur in this order:
•Machine authentication—ACS authenticates the computer prior to user authentication. ACS checks the credentials that the computer provides against the Windows identity store. If you use Active Directory and the matching computer account in AD has the same credentials, the computer gains access to Windows domain services.
•User domain authentication—If machine authentication succeeded, the Windows domain authenticates the user. If machine authentication failed, the computer does not have access to Windows domain services and the user credentials are authenticated by using cached credentials that the local operating system retains. In this case, the user can log in to only the local system. When a user is authenticated by cached credentials, instead of the domain, the computer does not enforce domain policies, such as running login scripts that the domain dictates.
Please Check the below link for active directory joining and managing users and groups
http://www.cisco.com/en/US/docs/net_mgmt/cisco_secure_access_control_system/5.1/user/guide/eap_pap_phase.html
