You will need them either all connected to the internet [every computer] and then host a lan and give everyone the IP to direct connect.
Or, you can try to join in on LAN, however I have never been able to get the ingame LAN to work without either direct connect [IP inserted through the MP server screen, not the LAN one] or using Hamachi, which will then sometimes allow it to appear on LAN, but I don't think you want to use Hamachi for something that big.
"Munchkin: One who, on being told that this is a game about politics and intrigue in 17th century Italy, asks to play a ninja."—
Andrew Rilstone