IMO your best bet is just paying for WiFi and using WhatsApp (or GroupMe). What carrier are you on? Some have specific "mobile-only" plans that are a touch lower than the full blown WiFi plan
Did some random Googling and found this article. Maybe one of these would be useful?
https://www.geckoandfly.com/22562/ch...-mesh-network/
Firechat looks most promising and I saw it mentioned elsewhere here:
http://mashtips.com/android-offline-messaging-apps/