To see an individual group sales order balance due, run the Group Sales Order Balance report. To see multiple group sales reservations, run the Orders With Balance Due Report

Alternatively, you can use query. There is not a field to calculate balance due, but you can output the order total and the amount paid. This can be exported into Excel and the totals calculated there. To do this in a group sales query: 

To output Order Total 
  1. Click Order in the far left column,
  2. Drag Amount from the center to Results fields to display
To output Amount Paid
  1. Explain Order in the far left column
  2. Click Sales order payment
  3. From the center column, drag Amount to Results fields to display