You can define the purchase groups in procurement basic data window.

As @Rasika Lakmal mentioned, you can define the purchase groups as in the above window. But you can not specifically segregate them, base on the buyers and suppliers. 

Hi,

Have you checked Purchase Groups tab in Supplier window?

However, this information is not retrieved for PO lines directly. 

And when a Buyer ID is connected to a Supplier, there is a combination of Supplier-Buyer-Purchase Group.

If you need to use it in any purchase process, you can consider using a Custom Event. 

Thank you.

Hi,

• You can define purchase groups on Procurement/Procurement Basic data.

• By entering a purchase group in Procurement/part/purchase part window, you can indicate a discount per purchase group when entering the suppliers. It should apply to the buyer connected in the purchase part record, unless a default buyer is defined at ‘Sites per user’

• The defined purchase group can be connected to suppliers in Supplier/Purchase tab/Purchase group sub tab.  This tab is used to connect and view the purchase groups of the specified supplier

I hope this information helps