Adding Domain users to unattend.xml

I have WDS setup with MDT 2013 on a Server 2008 R2 box. Everything is working fine but I'm having an issue figuring out how to add our Domain User group to the unattend.xml file so that it'll add it to the Local Administrators group. I know you can do this with a GPO but is there another way with the unattended install?

Thanks.