A person outside the game becomes your teammate. (Choose any number of cards in your hand, on top of your library, or on the battlefield under your control. Those cards become your teammate's hand, library, and permanents, respectively.)
1/19/2018 You and your new teammate become a Two-Headed Giant team and play under Two-Headed Giant rules from that point forward. Similarly, if you pick up a second teammate, you and your two teammates becomes a Three-Headed Giant team, and so on.
1/19/2018 Your new teammate must agree to play with you, although they don’t necessarily need to know how to play.
1/19/2018 Your new teammate must be someone not already in your game, although it can be someone who was once in your game but left.
1/19/2018 For game purposes, your new teammate becomes the owner of the cards you give them for their library, hand, and permanents. Remember to get your cards back after the game.
1/19/2018 You can give your new teammate Contraptions you control. You can also give them face down cards from your Contraption deck so they can form their own Contraption deck.
1/19/2018 You can choose any player not currently in your game. This includes people who used to be in your game but left it.
1/19/2018 It doesn’t have to be someone who plays Magic. This may be fun.
1/19/2018 If there’s no one around to make a decision, you can call or text someone if you need to. If there’s a physical action required, such as high-fiving, you’ll need someone there.