It That Gets Left Hanging

• WWizards of the Coast2018-01-19
  Official Ruling
  You must clearly ask for the high-five, although this needn't be verbal. The other person must clearly decline, although this too needn't be verbal.
• 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
  If there are no other people around to ask, It That Gets Left Hanging won't gain haste.
• 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
  It doesn't have to be someone who plays Magic. This may be fun.

Rulings via Scryfall