When I'm the last one to join I can see a "start game" button, but no matter how many times I click it, game won't start (everyone's action is green).
When I'm not the last one, the "start game" button won't even show.
My guess is that only the first one to join can actually start the game, since that person's got host rights, which goes aganist what is stated when you enter the game.
It has happened in all games I've joined.
Edit: rejoining the game when not in last position triggers the "start game" buttom to show, but it still does not work.