The nostr users I've followed use Signal for group chats.

no there isn't, many have tried making a group chat app nostr based but it just turns out to be a scam

You could setup a Matrix channel and even a server if you want.

Oxchat is used for group chat. Whitenoise is another latest app that is a nostr client but with Signal level privacy. It's still on beta though.

https://flotilla.social

https://chachi.chat

https://0xchat.com