I second, third, and fourth ! PI is the way to go. Use the search tool on here for the word intercom. You will find several discussions where many of us have extolled the virtues of PI.You will need a power supply/base station, headsets, and boxes, and XLR cable to interconnect everything. I suggest production intercom, they make great gear and you can pick it up much cheaper then telex or clear-com.
This is a link to production intercoms "econo-com" line, this is still bulletproof gear. http://www.beltpack.com/econo.htm
I have used those, they are OK until you use them for more then two hours and a ton of static starts appearing on the line. My personal rule with com, go wired unless you MUST be wireless. Also, wireless com is really only useful if all people can talk to everyone at the same time, and the master/slave thing is a bit annoying.I was really impressed with the products from Eartec at LDI. Unfortunately, I haven't found anyone who owns the products yet to give me a first hand account of how they work in the real world. But they have a 4 person wireless full duplex system for $800 (including belt packs and headsets no base station needed). You can't beat that. For About $550 you can add an interface that allows the wireless system to talk IN FULL DUPLEX with a wired system.