A free membership is the same as any other membership… the only difference is that the visitor doesn’t have to pay to become a free member and access the protected content that is available to this membership level.
For paid membership one will have to make a payment for the membership and then become a member whereas for the free membership one can simply register by filling in a member registration form.
The membership demo site has a good example. Have a look at the “Join Us” page from the following link:
A common practice is that you make your good stuff available to the free members but make the really really good stuff available only to your paid members.