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.

If there are no other people around to ask, It That Gets Left Hanging won't gain haste.

You can choose any player not currently in your game. This includes people who used to be in your game but left it.

It doesn't have to be someone who plays Magic. This may be fun.

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.