Our students keep getting removed from the user list (not by us)

Our students are having problems logging in to our wiki. They click on the email invite and then are taken to a login screen - when they try to do it, they are told they do not have access to this wiki, but they should because we added them on. Then, when we go back to check the user settings, their email address has disappeared from the list, so we add them again. Sometimes this works, sometimes it doesn't. What should we do to make sure they do not get bounced off somehow? Why do we need to add them over and over again? This is getting really annoying, especially when we have around 50 users on our account.
Thanks for your help.