I don't think you can do it....if I understand you...
Your users are using NT workstations to connect to the SQL server. They login to the NT using their individual login and password, but when they connect to the SQL Server they all use the same login and password. I would think that since the users NT login is never 'passed' to the SQL Server, you can't get that information from the SQL Server, it only exists on the NT workstation.
But, I might be wrong and we'll both learn something new.
-SQLBill