Go to the person, not the group chat about the person.

Talking around someone is easier and it fixes nothing. The group chat gives you agreement, sympathy and a version of events in which you are reasonable, all of which feel like progress while the problem sits exactly where it was. It also tends to reach the person eventually, in the worst possible form. A direct conversation is harder for ten minutes and cheaper overall.

Say it to the person first, in fewer words than you would use in the chat. Lead with what you want rather than with the complaint. Keep the others informed only where they are genuinely involved.