You will create the add-on first if you haven't already.
  1. Go to the Memberships tab
  2. In the Configuration section click Add-ons
  3. Click Add and name your add-on
  4. Choose the add-on type of Benefit or Additional Member
  5. Click Save
Now you will assign your add-on to the membership program.
  1. Go to the Memberships tab and click on the name of your membership program
  2. Select the Prices tab, and in the Add-ons section click Add
  3. Choose your add-on in the drop-down
  4. Enter the desired price
  5. Check the "Can purchase multiple" box if you would like the members to be able to purchase more than one
  6. Save