In most of the retail stores, there is always salesperson or salesman and owners usually want to track sales by salesperson, so that they can findout their targets, who has met the target and provide them incentives.
In Book Keeper, you need to go to Settings > Transaction Settings and enable Salesperson
Now salesperson field will appear while creating invoice.
If you want to display salesperson in view invoice (print/pdf/mail) also, go to
Settings > Template Settings > Header > Salesperson.
Once you have started creating sales, you can track sales in Sales Register report and filter by salesperson.
Here's video tutorial for your reference: