A. With a Pro system there is the option when editing the membership to indicate that the membership is free. If free, then you would need to see the id for the membership (look in the URL of the page when editing the membership and it will say ?id=*. The * is the id for the membership). Then, you would link to the join page using the link below putting the * for the membership and the USERNAME for the member that referred the new member.
~wmtsystemurl~/admin/join.php?user=USERNAME&membership=*###