File sharing – set permissions on XP Pro for limited

Can someone please walk me through setting permissions on XP Pro for file sharing with limited number of users? I only want one user on a separate computer in the network to be able to share….no one else. It’s a peer to peer network, computers are in workgroups by department name. It’s the same workgroup that wants to share. Simple file sharing has been turned off. I must be doing something wrong!