There are two ways to play with others, firstly if you are already game center friends: 

From the home screen tap “Multiplayer”. If your friend has already created a game you will see their match available. Tap “Join” and then select the available turtle you wish to play. 

If you do not see any available matches then you can create a new game. Tap “Create Co-op” and select either to start a new run (normally recommended) or you may continue your previous run. Select the Turtle you wish to play, in co-op you can not change your turtle. You Game Center friends will now see your match and can join you. 

If you are not Game Center friends then firstly tap “Multiplayer” and select “Create Co-op”. Once you are in the game you can tap one of the available + circles in the upper left corner below the avatar of your turtle. This will open the Apple sharing options. Select the App you wish to share with, e.g. Watsapp, and you can send a link to your friend. 

Your friend must have the game installed and be an active subscriber to Apple Arcade. Once they tap the link on their device they will load straight into your active game with you.