ntfs shares over a network

Ok here’s the drill.. I have 10 computers in a law office with 4 separate workgroups. Each workgroup needs to have their file shares confining access within each workgroup. Now The problem I am having is setting the permissions and finding the remote user in each workgroup and then granting access to that user. I am familiar with some aspects of permissions and users but most of the permissions grant access only for the local computer and not any other user on the network.. I cant “see” any other users other than local users/groups. So where do I go from here?