Are you talking about a Windows 2000 Active Directory structure? That's not possible...
You can set up Samba on Linux to act as a Windows domain controller, but it simulates Windows NT style domains, not Active Directory... AD is reallty nothing more than a fancy GUI to Microsoft implementation of LDAP services that went completely off the standards base...
Groups and permissions are very different on Linux than on Windows.. However, If you are looking for an excellent replacement/alternative to Active Directory you should check out SUSE Linux Enterprise Server and Novell eDirectory. eDirectory is similar in functionality to Active Directory, but supports clients of Windows, Linux, NetWare, Unix, etc.. (not sure about Mac).
It's not free.. but it's flexibility is great, and if this is for company use, it will be a great benefit. SLES also has excellent support, stability, etc.
Actually I just got a new whitepaper in my email today from TechRepublic. You have to log in to view the paper, if you don't have an account, make one. It's free and quite beneficial.
Ten Reasons for Choosing SUSE LINUX over Microsoft Windows Server
