First of all, whether you want to work alone or with others is a purely personal decision. Second, the best way to organize an outline, for most people, is to just go by the syllabus, or the same way as your case book. Syllabus is easiet and most sensible, in my opinion. Last, don't sweat that you haven't been outlining yet. If you start now, you have more than enough time, and if your school has a generous reading week, you will not even need it all.