Authenticate proxy non domain users

In our enviroment I'm installing a new Forefront TMG server. All domain users and computers can access internet but they need to authenticate themselves.
This isn't any problem and is working perfectly. In our company we have Microsoft Dynimacs NAV programmers and they use NON-DOMAIN computers but they have Active Directory credentials to authenticate to our servers. I want before they can access internet they need to authenticate themselve to our Forefront TMG so I can filter internet traffic. Also they computer need to automatically find the forefront tmg.
I found thos topic but it didn't really help http://social.technet.microsoft.com/Forums/en-US/Forefrontedgegeneral/thread/457e6db2-3fa9-4a6a-8a4f-c2f47eb3f119