The Special Offers: Buy X Get Y and Special Offers: Spend X Get Y add-ons in conjunction with the customer membership feature allows to provide the following special offers to your store member users:
Buy N items from specified categories to discount M of them (the cheapest ones). For example, if you set the discount to 100%, your customers will get the items for free.
Spend a certain amount in specified categories to get a discount on M of the items bought from these categories (the cheapest ones).
To configure the offers, open the Discounts -> Special Offers section of your store Admin area.
To create an offer that will be available only to the site members:
Create a special offer as described in the Special Offers section.
Scroll down to Conditions, and specify the membership level(s) eligible for the offer. Use the "Eligible membership levels" field for this purpose.
Save your changes.