Add or set up a Group by Box to allow sorting by Category:

  1. Right-click the word Categories in the task list column header (with column names or Arrange By).
  2. Click Group by Box if it is not already highlighted.
  3. Drag and drop the Categories column header into the new Group By Box above.
  4. If more than one Group By Box appears, drag the other one down to the column header and drop it there.
  5. (You can click the Group By Box to reverse the grouping order.)