For you to be able to give free access to your paid product, create a coupon that gives 100% discount to your product. They will just have to type in the coupon code to get your product for free.
To do this:
On the left-hand side of your screen, go to the Brand Builder tab then click on the Manage & Scale tab. Once that is expanded click on Discounts & Coupons. Next, click the blue Add New Discount/Coupon button. Once the next page pops-up, make sure that the Coupon option is selected at the very top.
On the Coupon creation page, fill out all the information needed. Make sure that on the discount type, if you will choose percentage, just type 100. If you will choose amount, just type in the full amount of your product and set the currency on the next field.
**IMPORTANT**: Make sure to click the Create Discount/Coupon button once you're done setting up to ensure it saves.
After creating your coupon, all you have to do now is send the coupon code for the coupon you've created to people who you want to be able to redeem it.