Found a way to report them in game as well.
When they invite you to a group, do a /who for their name. Send them a whisper. They won’t respond, but you can then right-click their name from your whisper, which brings up the report menu.
I didn’t get an in-game confirmation, like a ‘thank you for your report’ kind of message, but I don’t know if that’s intended for the Classic reporting system.