To change the group members for a submission that has already been submitted you need to allow the students to submit again and then have them submit with the right group members.
The way you allow students to resubmit is different for a Live Session and a Homework Session:
- If your assignment is a Live Session, then follow this guide: http://help.peergrade.io/how-to-s/assignments/how-to-allow-a-student-to-re-submit-in-a-live-session
- If your assignment is a Homework Session, then follow this guide: http://help.peergrade.io/how-to-s/assignments/how-to-allow-students-to-submit-work-late
Allowing students to resubmit will not delete their work or the feedback given.