There are multiple options. Simplest is probably a summary report where you group order records by sales person and use a summary field inside a sub summary layout part to compute the sub total. Put the same summary field in a trailing grand summary part and you have your grand total. And such a report can list the individual orders or just the sub totals by sales person.
What such an approach would not do is list the sub totals a second time after the grand summary. Such a "summary recap" can be produced in a trailing grand summary part with either a portal or a calculation field using ExecuteSQL(): FMP 12 Tip: Summary Recaps (Portal Subtotals)