By default students always give feedback individually no matter if they have submitted with a group or not. However, it is possible for the teacher to enable group-to-group feedback and to allow students to provide feedback as a group.
If the teacher has enabled group-to-group feedback students will be given a notice on their Review page that they are giving feedback on behalf of their group.
Only one student from a group can give feedback at a time. If multiple students access the Review page simultaneously it will be locked to the first student accessing. The other group members will be given a notice about who from their group is able to review.