I currently have an online shopify store which sells to members only. Currently members can sign up through the website. What i want is to add an affiliate type system where people can only become members if they have been referred by existing members only. Also, the number of new members that existing members can invite will be limited to a certain amount a month. I will be able to control as much as possible such as the amount of possible invited per month etc. Also, when an existing member invites someone and they sign up and become a new member both will get a $10 (or whatever) discount.