Better Than One

• WWizards of the Coast2018-01-19
  Official Ruling
  It doesn’t have to be someone who plays Magic. This may be fun.
• WWizards of the Coast2018-01-19
  Official Ruling
  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.
• WWizards of the Coast2018-01-19
  Official Ruling
  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.
• WWizards of the Coast2018-01-19
  Official Ruling
  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.
• WWizards of the Coast2018-01-19
  Official Ruling
  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.
• WWizards of the Coast2018-01-19
  Official Ruling
  You can choose any player not currently in your game. This includes people who used to be in your game but left it.
• WWizards of the Coast2018-01-19
  Official Ruling
  Your new teammate must be someone not already in your game, although it can be someone who was once in your game but left.
• WWizards of the Coast2018-01-19
  Official Ruling
  Your new teammate must agree to play with you, although they don’t necessarily need to know how to play.

Rulings via Scryfall