Someone can provide step by step instructions?
I am a novice in Obj-C and social aspect of gaming.
I’m novice with Obj-C, but what you need to do is to write a wrapper C++ class that will execute objective C code in it’s implementation (.mm file).
See how they did it with the audio, is should be similar.
See this (and download the zip file I posted, last comment):
Should help a bit, then follow the Apple iOS docs https://developer.apple.com/devcenter/ios/gamecenter/