It’s not easy to use group chats with users who not use DC.
So I think it would be helpful to add a hint to every group message.
My idea (if it’s not too hard to implement):
For group messages replace the standard signature with a signature like this:
“This is a message of a Delta Chat group.
Please use the “relplay to all” button of your mail client. So every group member will get your message”
And also you can add a link to the DC website (https://delta.chat/de/download)
Only non DC users would see this hint