How it works
A group chat is a crowd: lots of people see what you send, and some of them you hardly know.
You can be added to a group you did not choose. Look at who is in it with a grown-up, and leave if you want.
A private message is only for the person it was written to. A screenshot of it, or forwarding it, is not yours to share.
When a crowd laughs at someone, you do not have to join in. Write kindly to them on your own.