When I use test flight for my app, and I want to test my Game Center capabilities, will I have to make sandbox accounts for all of my test users and instruct them on how to use those accounts, or can they use their normal accounts?
They can use their normal accounts, but they have to have sandbox mode turned on under Game Center in their settings. This will create a new sandbox account for them using their normal account.
Apples developer guide says: Important: If your app uses Game Center, testers will be required to turn on the Game Center sandbox from their device settings in order to use the Game Center features while testing the app.