If by "hardline" you are referrng to UTP/Cat{5,5,e,6} then all you need is a switch, probably a 10/100 switch (that's about all that's available these days).
The existing cable would go into the "Uplink" port, the other computers would go into the "plain" ports.
Keep in mind thast you are sharing bandwidth for any resource at the other end of the existing cable (Internet, Server, whatever) ... instead of ~100mbps that you got from the single existing cable, if you are both trying to stream (either up or down) to something at the other end of the existing cable, you will be limited by the existing bandwidth (~100mbps) divided by the number of active clients.
If the other end of that existing cable is something like a cable or DSL modem, you will be dividing that bandwidth as well (by the number of existing users) ...
If the application that you all are using is not streaming (i.e., downloading files), then the natural multiplexing effect of Ethernet shouldn't affect your available bandwidth too much.
If you are just passing files between the computers connected to the switch, then the information will not be sent down the existing cable (there'd be some administrative traffic, but that's it). That is the nature of the switch versus a hub (which would sent the traffic down every live cable connected to it).
Good Luck
Scott
