Sales Rebate setup for group of Customer

Hello,

I have a case where the rebate need to be setup for a group of customer. The customer belong to a group and they all participate to the rebate threshold. So I've setup a group composed of a cat code (from Customer master). But it seems that when the sales order is done for the rebate. The open amount does not include the sales order that are done in other customer of the group. It is like the threshold amount works unitary for the customer and does not apply at the group level.

Do you know if it is possible to set a rebate at a customer group level ?