Password Based Event Check-In

I am looking for a way to track attendance of events by having the user enter a password that is posted at the event. For example, if they come to a seminar, the password will be posted on the projector, then all they have to do is log into their wordpress account on our site and enter the password in a page. Then their name is stored in a file so we can track who attended. Does anyone have any idea how I could do this?