Hi
Inorder to have group of people as user Roles, there is a tool called 'Active Directory web service' which helps you to add all people in one group which can be accessed from the User Roles.
Look the the following which gives you the details about the Active Directory web service
http://www.qdabra.com/proddetail.asp?prod=QADWS
Madhuri
http://www.ggktech.com