The functionality requires the Volume Discounts add-on to be installed and enabled.
A store admin can set up membership-specific discounts. The process of setting up a membership-specific volume discount is relatively straightforward:
Add a new offer on the Volume Discounts page of your store's Admin area. See detailed instructions here.
Specify the subtotal amount starting from which the discount will be available. Set the discount amount (select an absolute value or a percentage) and select the membership level that a customer must have to use the discount.
Save your changes.
More info on volume discounts you can find here.