On the site members page (http://adultmembersites.com/app/#/members-active), click add free member button:
Search for member: If you feel that the member may already exist in the system, first search.
If the user is not in the system, create a new user by entering their username and email:
Select a free membership length & click "CREATE FREE MEMBERSHIP:
The member will receive one of two auto emails based on whether or not they are already in the system or not:
These auto emails can be customized here: http://adultmembersites.com/app/#/auto-emails