Currently I'm using TestFlight to push beta versions. If I change the provisioning profile (by adding a new user's deviceID) do I need to then push a new build, or will the new user's deviceID be automatically recozniged by the existing provisioning profile on the app. I'm guessing the former...
It's the former. The provisioning profile is bundled with your app and there's no way for iOS to get the new provisioning profile otherwise.