iOS 8 beta 5 Game Center Sandbox won't recognize my app

You should turn on the GC sandbox mode.

go to settings --> game center

scroll to the bottom, there is a "sandbox" swith under "developer".

(Important! - These developer options are hidden until you have connected the device to XCode 6. If you didn't use XCode and deployed your app in some other way - e.g. Testflight, iPCU, iTunes, then you will not see these options)

If you aren't seeing the sandbox switch under Settings > game center (developer section), then you need to connect the device to XCode

OR: Use Apple's testflight iOS app found here on the App Store: https://itunes.apple.com/us/app/testflight/id899247664?mt=8

That will enable the developer Sandbox switch as well. Using the old testflight web link tool on iOS doesn't enable the sandbox switch.