Restrict app download/registration

Hello Ionic and community!

My cousin is getting married in a month and I decided to make an app so that friends and family that assist to the wedding can use the app to take photos which will be automatically be uploaded to firebase storage.
My doubts aren’t ionic 2 related directly but I’ve used the ionic 2 framework to build the app in 3 days, the performance in prod mode is good and the native-like feeling is spot on, so thank you!
I am posting my doubts here so that other ionic 2 users which have similar doubts can find an answer to.

The app is only intended to be used by friends and family but I would like them to easily download it through Play Store for Android and Apple Store for iOS.
I wouldn’t like other people seeing the photos we upload or uploading their own and eating all of the firebase storage quota.
Restricting downloads on the stores is quite complicated for what i’ve read. Also I dont know the email of every person asisting nor I want my cousin to gather them all. So once we are at the place I will tell people to download the app.

One idea is to give people a URL only I know so that they register and then get redirected to download the app that will only have a login page as they have already registered at the website. How does this sound? Maybe Apple doesn’t like it and doesn’t approve the app…

Can anyone think of better solutions?
Has anyone encountered a similar situation?

Thank you all. Cheers!

One possibility might be to use geolocation somehow to limit access to people within X radius of the wedding venue.