If students post to a group discussion before they are placed in a group set, their discussion posts appear on the main discussion topic rather than in the group discussion. During add/drop period, teachers often wait a few days until enrollments are finalized before assigning students to groups. Students who work ahead without waiting to be assigned to groups then end up posting to the main discussion and get confused when they are eventually assigned to groups because they lose their original post. We have been advising our faculty to unpublish group discussions or delay availability until they have the groups sorted out, in addition to leaving very specific instructions asking students to wait for groups to be set up, but would like a more permanent fix.
Another related issue occurs when a student is added to a course late, as the instructor may not notice right away and therefore doesn't add them to a group. The student then posts to the main discussion without being in a group. The grading experience is compromised and it confuses students.
If a discussion is set up to be a group discussion, prevent students from posting unless they are in a group. Display an error message to students not in a group and alert instructors that there are students in the course who still need to be assigned to a group.