NFS introduces a fairly different approach to normal PC networking. Most PC file sharing protocols establish a relationship 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 sense.
However, 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 machines on a given network. Once an NFS relationship 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.
In fact, one user on the client machine will have about the same access as if he was logged into the server. His access will be limited to the different parts of the filesystem exported by the server.