Hello Sid,
You can simply use there Unix Name into the Sudoers definition, as you are using Express their Unix Name will be equal to their SamAccountName from AD.
For example if you have two AD Users fabrice@domain.com and sid@domain.conm who are login onto your server and should be able to get Root privileges as SysAdmin, you can add the following straigth to your Sudoers file:
User_Alias SysAdm = fabrice, sid SysAdm ALL = (ALL) ALL
Of course this a very basic example, but I am sure you get the idea.
Remember than in doubt you can see what is the Unix Name of an AD User by running the command below
adquery user -n fabrice@domain.com
Or simply (assuming you are using Linux)
getent passwd fabrice@domain.com
Hope that helps,
Fab