Microsoft Knowledge Base Article 303973 contains the following summary:
This step-by-step article describes how the Pre-Windows 2000 Compatible Access group is used,
why it is needed in a mixed-mode domain, and how to set up the group by using the Active Directory
Users and Computers snap-in and command line.
Setup procedures that you run in the graphical user interface (GUI) are done by a member of the domain administrators group on a domain controller (DC) that is running Windows 2000 in a mixed-mode environment.
Setup procedures that you run at a command prompt are done by a member of the domain administrators group on a DC that is running Windows NT 4.0 RAS (or RRAS) server in a mixed-mode environment.