The solution is simple:
1. The group chat initiator name a password( you can literally divide it into several part or just as whole)
2. sha256 the password, and you get the hex private key
3. log in and DM yourself
Discussion
btw:
It can be made into a kind of inner login feature build in client, the group initiator can give every chater a npub icon or avatar that can differentiate anyone is talking, in making it not like talking to oneself (this is an option feature).
For making the newcomer can't read the former message ( I think this is crucial, cuz it's the CORE feature of WeChat groupchat, and in telegram, you can read the message before your joining, that nostr have already solved ):
When every time newcomer came in, the chatroom nesc changes ( automatic, UI senseless ), like HD seed or Merkle Tree, and so you can not read the message before, but former people can read the following message.
