You need to be more specific. Used operating system (number) mostly. You find that under the “About this Mac” in the first entry of the first menu in the menu bar.
Also read the tutorials and have a look at the videos Alex has provided. It might look a bit different now, but basically that’s the way to go.
If you want fast communication without a server also have a look at the port forwarding, since all (but one) member of the group needs the port forwarding (except if you are in a public network like in a university or bigger company).