Answers
When adding photos to a person using iOS 8, the screen freezes.
Using iOS 8, there is a problem adding photos from within the iPhone app. You can still add photos using Reunion on your Mac (and those photos will be sent to the iPhone).
Thus far, this is the only issue we know of using Reunion on devices running iOS 8.
If/when your device's screen seems to freeze, double-press the physical button on the front of the device to bring up the App Switcher. From there you can swipe the preview of the Reunion app upward, which will close the app. You should then be able to go back to your Home screen and re-open Reunion.