It's worth putting a fine point on what you want to happen. If you have 50% "no shows", there are two main options:
1. More accurate RSVP lists, but fewer total attendees. Putting up barriers to RSVP (ticket prices, deposits, shaming/consequences for no show). People will think more before clicking the button. And while your list will be more accurate, you will scare off people who otherwise might have come.
2. More total people, but inaccurate RSVP lists. By keeping the bar to RSVP low, you'll get people RSVPing that are on the fence. And some fraction of those people will show up. Your total numbers will be higher than the other option, but you'll have a ton of no-shows.
But there's no version where you get 100% of those RSVPers due to a change in app functionality.