Ok. Tell all of your friends to bring one RJ-45 cable OR their wireless card/adapter. You are going to need a wireless router. Have yourself and all of your friends with wireless cards/adapters configure their computers to connect with the wireless router. All of you friends who don't have wireless, use RJ-45s. Plug one end into the RJ-45 port on thier computer (it could be on the motherboard or on a seperate ad-on card such as a NIC) and the other end into the router (it should have 4 ports for this). If you are going to have more than 4 wired computers, you will need another switch and another RJ-45 cable. Plug one end into any port on the switch, and the other end into the router. Once you are all connected, host a game and have all of your friends join it.
Phew. That was a lot.