That might be more of a Windows/Active Directory question than SQL Server, but this might help:

http://technet.microsoft.com/en-us/library/cc755692(WS.10).aspx

In our client's case it was to do with being able to grant database access...

Just solved an issue like this here.

Our client had a group in AD mapped to a SQL Server login, mapped to a database user. For various reasons, they decided that...