>>The apostrophe should not be in "attendance's." It is a possessive, i.e. something belonging to attendance, i.e. gibberish.
>
>Basically we want to show how many people are registered for the Event. What should be the exact phrase using the attendance word?
>
>Thanks again. I agree about possessive, but it was suggested by a native speaker for some strange reason.

Illiteracy is not strange. It's domestic ;).