Membership Plugin - Member Invite Code Only Setup ,w/ Free Subscription Setup

Hi, i just installed membership and buddy press , I require ALL users that register to already have an invite code I just need a step by step on how to make it work with a Free Subscription based system , I dont want them to pay for subscription because I have already given them a code upon giving them a Thankyou card , each user has a unique number code such as 3647823 .
I would like them to be able to only register with a valid number on the card ; I have entered the list of numbers into the Extras section of Membership , however i do not see the Signup Button nor the field for signing up am i missing something or is there something wrong , incorrect with the setup ?
Is their anything else i need to install ?

Thanks in advanced
TH