Whatever you say about someone in a group, assume it’ll reach them.

Group chats are not private and never have been. Screenshots, forwarded messages, somebody reading over a shoulder: there are a dozen ordinary ways for a line to arrive in front of the person it was about, and most involve nobody intending any harm. Assume it will reach them, because that assumption is usually correct.

Before you type something about a person, check whether you would be comfortable with them reading it in that exact wording. If not, either soften it or keep it out of writing entirely. When someone else starts it, you can let it pass without joining in, which is noticed more than you would think.