Sharing Files from Local Account to Domain

This is a really dumb question, and I don’t even really know how to word it, let alone google it.

I have a client who has just migrated to a domain from a p2p workgroup. He has decided instead of using domain accounts to leave the local accounts for some of his users

Here is the question: When the users try to access the server from “My Network Places” some users are given the option to input a domain username and password. Others are simply denied and told that “they may not have access!”. How would I make it so the file triggers a prompt for username and password instead of just denying access?

The server is a Server 2008 SBS, most of the user computers are XP’s.

Thanks