Excel formula using nested IF statements

I need a MS Excel expert to write a nested IF or SUMIFS formula to build a dashboard from a large data set of sales tracings. See attached pdf and spreadsheet. I have sales data that i wish to isolate how many unique customers drive the orders. I currently have the formula o calculate this desired number for the entire column but I need to add an additional criteria (Salesperson ID) and calculate the number of unique "Cust ID's" associated with each Salesperson ID. Take a look and let me know.