Sounds like this is an authentication issue. "A user account defines the actions a user can perform in Windows. On a stand-alone computer or a computer that is a member of a workgroup, a user account establishes the privileges assigned to each user. On a computer that is part of a network domain, a user must be a member of at least one group. The permissions and rights granted to a group are assigned to its members."

So create a domain or workgroup environment and you should be able to share the files with the security settings you desire.