Add new products not currently listed as inventory using inventory setup (admin menu > inventory > inventory setup).
*Note, when receiving new stock of an existing item, this will be updated in the Inventory transaction window NOT the inventory setup window.
When a new product is added in the inventory setup, it will automatically add this item to the price list for use on invoices. Also, if a new item is added to the price list, it will automatically get added to the inventory setup list of products.