Chaining together a few WiFi routers and running a local IRC server with ergo could work well (so get everyone on the same WLAN even if no Internet). There is also Briar chat on mobile. Maybe Mumble for voice could work. I'd look for routers on Craigslist or eBay that are under $20 each and have OpenWrt support. Have had good luck with the Netgear R6220 in the past, but there could be other good options as well.