The LDAP user directory plugin hooks up existing LDAP directories with the internal FarCry security model. Active Directory is well supported through the Active Directory LDAP interface.
Setting up LDAP and Active Directory (via the LDAP interface) is relatively straightforward. Unfortunately, there is no "standard" configuration for LDAP and so you might need to do a bit of detective work to sort out the appropriate configuration details.
Most implementations should be covered using the standard config provided. There are also a number of potential extensions you could apply for more complex scenarios.