In the initial view of the Grade Centre, students are listed individually but when you click to view the attempt, it is the group attempt that you are taken into.
You can enter a mark for the whole group using the pink attempt field and add feedback for the whole group using the Feedback to learner area. Submit to save your marking.
You can adjust the group mark for specific students, for example if they contributed less than the other group members, by using the pencil icon (override/revert), entering the mark you want that student to have and then clicking on the green tick to save.
All members of the group will see the feedback that you add under Feedback to Learner. If you want to add individual feedback, locate the specific student and from the drop down select View Mark Details and then Edit User Mark. The feedback you enter here will only be available to that specific student. The other students will just see the group feedback.
