PopIn is a free event app I'm building, now in early access. The one rule behind it is that your guests never have to make an account. You create an event, invite people by email, text, or a link, and they RSVP in one tap with nothing to sign up for or install. As the host you get a live guest list, recurring events, and one update you can send to everyone if the time or place moves. There's a small social side for people who do sign up, but it's kept light on purpose. I'd love early feedback while I'm still shaping the roadmap.