This could be implemented with something similar that a creating a review board. But instead it would be playable between yourself and one guest user
The person given the link is offered the option to accept the challenge without registration or login
Once someone has accepted. Cookie is created for that game in case his computer run out of battery or something
And noone else can accept the challenge
Once the game finishes
The user is offered to create an account to keep this game in his game history
Should he refuse the game with remain as an invitation game history for the user that created it.
Seems nice and easy?