So, another issue from my online class.
I have created groups in the class, with students assigned automatically. I discovered, however, that students who joined the class after the groups were created (during the drop/add week) were not placed in groups -- so they were able to do their discussion postings in the "root" discussion page (where the prompt is). Is this a bug or a feature? I would think that lots of schools allow students to add classes after the class has started, so Canvas should automatically place new students in groups.