I had a similar thought as well, except no automatic sign up. What if the sent email had a reply-to address specific to the event?

In the event invite email you could have a short message that tells the person can just reply to the email with a 'Yes/No/Maybe' in the subject and any thoughts or questions in the body. The service would then show the appropriate response on Julie's event. 'Maybe' responses would be marked somehow to show they need manual review and Julie would be able to see the the response text from the email and easily set the status for that person.

To help make it simpler, for people that don't change the subject you could default to a 'maybe/review' response or try language processing on the response text.

Combine this with your idea of including links in the email for a quick response and I think it could work.