I would like to grade each student and therefore have all of that student's posts in one place for grading. How do I arrange that?
When setting up the discussion, you will want to choose GRADED in OPTIONS settings. Use Speed Grader for the discussion to view all of the interactions (posts and comments) for an individual student in one place. You will also find a link to view a student's posts and comments in context.
This guide might be helpful. It shows how discussion grading works.