assisn receipt numbers as passcodes

I want to offer my customer access to a private page where they can post prayer requests. I want this page open to only those customers who have signed up for my year long prayer study. Each customer needs to have their own unique passcode so if they post something offensive, I can remove only them. Is there a plugin that assigns a unique receipt number that could also be used for the passcode, so once they sign up they can join the private prayer page?