How To Find Group User Login Name

Our system administrator set up an NT server group in order to allow
our users to login to our application via https to our sql server.
The group appears as a User in SQL Server when you look at it in
Enterprise Manager. That said, I can not see the users associated
with the group from Enterprise Manager, but know they can login to the
database.
The problem is this. When we login via the web we get access to the
database without problem, but when you look at the current_user what
you see is the login Name the user entered and NOT the name of the
group/User. That is to say, I can see a UserID which is not listed as
a User in SQL Server and can't see the name of the group, which is
listed as a user in SQL Server.
I need to know who's logging in order to direct them to the
appropriate web page via their role. Before the admin set up the
group, I was using sp_helpuser to get the role, but then again I had
the userID to do this.
The question I have now, is there any way to see what the
group/user is who logged in i.e. the goup listed as the User in
Enterprise Manager? Otherwise I have to build a table of userIDs and
their group/User name, which seems to defeat the purpose of having he
server authenticate users.

Thanks,
Tom