Will SYSTEM_USER return the unique user id for users authenticated by Active Directory
A call to system_user will return the AD user's name in your case.  For instance, if you have a user set up as YourDomain\YourUser1:

select system_user

-- this returns "YourDomain\YourUser1"


If YourDomain\YourUser1 is part of a AD group (i.e. YourDomain\YourGroup1), then it would still have the same output as above (returning "YourDomain\YourUser1").