Can you replicate the error by plugging their email addresses into the form? Without that, there’s no way to know if they are entering typos. Something like joe@aol without the .com or omitting the @ symbol or including a space in the middle of the email address where it’s supposed to be a – or a _ or any number of other glitches.
You would think most people would know how to enter their email address into a form, but I’ve run into plenty of unsophisticated computer users who don’t. Older folks, sometimes, or just people who aren’t good typists. I’d imagine an event at the library could draw all types.
I’m not aware of this complaint occurring on a regular basis, and it hasn’t come up on my own sites that use RSVPMaker.