How to provide web access to domain user through a non-domain ISA server?

We’ve an ISA server in between the internet and the domain controller which acts as a firewall. The ISA server is not a member of the domain. all the machine in the domain are restricted to the internet in a way or another. Now i wan’t to give full internet access to some of the domain members, who can log into any workstation and access the internet. How do i do this?