Well...since your running in a workgroup with no domain, there is no such thing as a network user in that case.
The other major problem is that you're talking about restricting access to computers, but CIFS (shared folders) and NTFS (filesystem) permissions only deal with users, not computers. (While it's true that you can add a machine object to an ACL, that only works in a domain, and it only grants access to the Local System account for that computer, not the entire computer and all its users.)
What you can do is to create a unique account for each server (but don't use the server's actual name, which isn't supported). You will need to create this same account (the exact same account...same username...same password) on both the source server as well as the destination server. This will allow that user to authenticate. You can then assign that user to any share/disk permissions (you must configure both) to allow him access. Repeat this for each server that needs access and youre getting close.
Now here's the catch. Remember how I said permissions are user based, not per computer? What this means is that whatever is attempting to connect to the server will need to be running as the user you created above. So the access only works for that user, not the entire computer. That's the only way I'm afraid.
Your other option (better imho) is to implement a domain and use Active Directory to assign permissions. But this still won't give you "computer-level" permissions, only users.