Alternatively, you can create a Running Total Field on your group title (use "count" or "distinct count") that resets on a change of your group title. In your example, you'd be receiving 3 rows of data per group, each with the same group title (e.g. "Group A"), and the RTF would count 'em 1, 2, 3, and reset the counter upon every change of the group title. Then just drag your new RTF onto your report where you want the number to appear (left of your Qty in your example).
Hope that helps.