My iOS app is currently on beta in TestFlight, and as a way to retribute to the nice people who helped me test it I would like to offer them some goodies such as, for instance, the full final version of the app for free.
For this, I was thinking of sending them a last beta version which would, automatically and upon execution, store some kind of ID from the logged in user into a VIP list I would keep online and then every next version of the app would check for their ID in order to verify if the user is a VIP user and unlock all the premium features.
Is it possible to obtain in the Cocoa apis a unique identifier that is associated to the user (as opposed to the device unique identifier)? I want this because I want to recognise the user in whatever device he installs it. I would like to avoid having to make my beta testers manually register as VIPs.