I have not gone all the way but from my looking it looks like all parties need to meet and make a multiplayer game and save it.
Then as each opens the game, logs in, plays, saves, email.
The problem would be if anyone changes the game in any way durring playing. I do not know how much it would take for the save to not work.
If they could just add passwords for each player as optional then 2-4 person games would be posible.
I have also wondered if a game can be made on a server and the server game is always on the GC multi page. From here a person can join the game and only play 1 turn. If that would work then we have another viable option. Just no passwords.