One candidate said in the chat that she was not made aware that this was a group interview. I went back and looked at my emails and none of them mentioned it at all; in fact, they made it sound as though it was going to be a team from the company interviewing one person.
Other people said in chat that they had taken off work for this interview and that their time had been disrespected, and that it should have been disclosed that this was a group interview. The owner of the company said (what he had said earlier) that this must have been a mistake by the recruiting team.
I said that it was interesting that this mistake had been repeated almost thirty times, and that I was no longer interested in the position. It was infuriating that this happened, but also awesome to see so many people stand up for themselves and leave. Hopefully this employer will realize that he can’t get away with this any more.
Update: I was just sent an email containing the next step in their hiring process. I left after saying what I said in the paragraph above. Impressive levels of incompetence here.