As a guest you can easily join without registration! The first thing you'll see after opening the app is a white bar where you have two options.
Click the QR icon on the right and the app will ask for permission to access your camera. This is necessary to scan the QR code. Once you see your camera, hold it steadily towards the invitation QR and you'll be redirected to the event.
If you have an Event ID, you can type it directly into the white text field to join without the QR code.