After running a report, right-click on any column header to display a pop-up menu of column options available for that column.  (Note: The options will vary depending on the grouping state of the column.) Select the "Group this Column" which is the third item on the pop-menu if the column is not currently grouped.

The following is an example of a column pop-up menu.
There is an unlimited number of columns that can be groups. The following is an example of three levels deep: Item type; Item primary; and Item Sub.