Here's a guide to Apple Arcade Game Center: https://support.apple.com/en-us/105022
and here's a guide on how Family Sharing for Apple Arcade works: https://support.apple.com/en-us/108380
Firstly, the easiest way to find people is to select "Challenge Friends"
> If you're on Bluetooth + the same WIFI, other players playing WHAT THE CLASH? nearby will show up in the "Challenge Friends" overview!
There is also a code link system in the game!
If you want to send a code
From the main menu
> Select "Challenge Friends"
> "Create Game"
> Copy the Invite Code and send to whoever you want to play against!
If you get a code from someone
From the main menu
> Select "Challenge Friends"
> "Join Game"
> Paste the Invite Code and start the match!
Please note:
You need two seperate Game Center accounts to play against each other - so if you're logged in on the same account on two different devices, that won't work.