How do I grade a participant differently from the rest of the group?

Firstly, complete the marking for the whole group on one of the submissions when you hit ‘grade’. Then scroll down on the right-hand side of the window to find where it says, ‘Group submission settings’ and the response for ‘Apply grades and feedback to entire group’. Select ‘Yes’ and then click save changes.
This will then apply the same feedback to the entire group.

Then find the student that needs to receive a different mark and complete their individual feedback. Change the previous response to ‘No’ so it only applies the individual feedback to the one student. Then repeat on a different student if necessary.