NFS introduce a fairly different approach to normal PC networking. Most PC file sharing protocols establish a relation between one user and one or several servers. The machine used by the user is not relevant. Given that many PCs are running single user operating systems this scheme make sens.
Unix and Linux are multi-user systems. NFS configuration is a machine to machine issue. One NFS server is granting access to different machines either one by one or to all machine of a given network. Once an NFS relation is activated between one server machine and one client, all users on the client have access to the server, in the same way they have access to files and directories on the client machine.
If fact one user one the client machine will have about the same access as if he was logged in the server. His access will be limited to the different part of the file system exported by the server.