I’m not really an expert or close to that but I believe some apps (like whatsapp) encrypts each message individually for each member of the chat group. While the server relays messages, it does not have the private keys of recipients. That should work, no?
I agree that this is not a technical issue though. Still there are possible measures to be taken.
I agree that this is not a technical issue though. Still there are possible measures to be taken.