I imagine it will be like this: You say /highfive [playername] and the player that you typed in will receive a message saying if you would accept or decline the high-five. (This is to prevent exploits a little better.) If they decline, the high-five messenger will receive a message saying that the player has declined the high-five invite. If they accept, however, they will be teleported in front of the messenger, and play an animation involving the hand, which I imagine will be hard if not impossible. Who knows, though; I can't script for stuff.