People that you already have in your phone contacts are automatically available for free as you already have their phone numbers. Correspondingly, the same applies to users that have your phone number.
There are several ways for you to add a user to your free list and manage it:
1) Access the profile of the user you want to add and you will have an option to add that particular user to your Free list. Simply click on the "Request free" button appearing on their profile to request the ability to call this user for free.
2) Another option is to go to your Settings tab and look for the Contact section. Here you will be able to manage your Free list by clicking on the "Free" tab. You will be able to remove users from your list easily.
You can add or remove a user from your Free list whenever you want but remember you will at times need to request others to accept your request.