Just wanted to say TestFlight is amazing. It'll help with this. You still have to provision the device id, but you get the device id automatically from them when they install the testflight app. You send them an invite, they can then install and use the testflight app to see what the latest builds are they can test. You can also send out notifications when you upload new builds, and watch in realtime as people get notified, open the email, and install the new app, all w/o changing your app's code. If you import their library and make a couple minor changes to your xcode project you can also see metrics of when users are running the app, or use it to gather feedback from them in-app.
All in all it's really a great offering.