Simple NOS question (I Hope)

I cant believe I'm posting this question because i bet its so easy, But some times you have to bite the bullet and feel stupid.

I'm trying to learn about network operating systems and how they relate to a network. I understand that in a P2P network each peer acts as a client and server, and that when a host on the network has a NOS, then this becomes the server. What i don't understand are the functions that the NOS implements. I believe one such function is deciding who can access the network. How can the server do this?? The server is just one of many machines connected to the cable. Surely each host still has access to the other connected hosts (The server machine isn't acting as a switch, with each host connecting to a separate NIC on the server. What is to stop machine "A" communicating with machine "B", or any other machine???
I know a little TCP/IP and the OSI, although NetBui and NetBios still confuse the hell out of me (I do, however, understand DNS).

Any help greatly appreciated.
TIA.
Stuart