Signing up is not the same thing as enrolling. Enrolling takes time and thought. Signing up takes only the swipe of a pen or the click of a button.
Perhaps part of the solution to backing out is asking for more on the front end than signing up. Because if signing up is easy, then so is backing out. Enrolling is more involved. For that you have to show up, maybe stand in a line, put some skin in the game, be introduced to the other enrolees. Maybe the form is paper and not a website. Maybe you have to mail or carry it in. Maybe the deadline is fixed, not flexible.
For some things, sure, signing up is all you need. Sign people up for a pot luck or a shift at the fundraiser. That works, and Signup Genius is invaluable. But for a retreat? A mission trip? Those are bigger commitments. You want people enrolled more than simply signed up for those. Maybe it’s worth asking people to do a little bit more.