Hi,
It depends your multi player engine, but typically you'll need a way to sign in players, cause you can't rely on any networkId to refer to players data like scoring, only a proper user management system allow for this.
typically, one approachable system is Parse, but it's expensive if your usage goes above the free tier, so I would look for game hosting systems like steam or some proper back end for games that provides everything ( user management, game center like scoring, friend list, etc etc).
Bye,
Jean
