It's impossible to track commissions by sales representative for now, commissions are calculated on products sold only. 

When you create a product, you can enter a % of commission directly in the "Commission" field. If you enter 5% commission for a sale, the system will automatically calculate 5% commission on every sale you make. This commission will be disbursed in your cash flow budget at the same time that you cash the sale.

When creating a new product enter the commission percentage for each unit sold in the appropriate field.