Hello, I would like to to create the attendance register app you need. Although you're asking for a native app my proposal is a web responsive app, my guess is that this way you don't need to require your attendees to install a new app on their phone just to register. If you're interested this is what I can provide:
1)Pre register responsive form. In advance of the event the user can register herself. in return she will receive an email with the confirmation including a QR code that can be printed or just brought on her phone the event day
2)On site register form. To be used on the event day to register people on site. It will also produce an email with the QR code but can also be printed to give it to the user
3)Check in app. On the entrance it will read the QR code and validate it.
4)Admin app. Where you can see the list of all the registrars, also the attendance after the event. create reports and export information.