In-VPN P2P group messaging system

Hi guys,
I would like to make some kind of a messaging bot that can send the data gathered from PLC (I have the data already so that's not a problem) however it would need to run inside of a VPN with no internet access, so the messages would need to be send locally and not somewhere in the cloud. Moreover the users would need to have groups to which those messeges would be sent to. Maybe there is already a p2p messaging system with groups that would work with node-red? Any direction you could point me to would be extremely helpful!

I'm not personally aware of something ready-built but if you can live with a web interface then Node-RED certainly has the tools to do that.