I think u should be able to choose how many players can join your game when u make new game. Just in case you want less people in game. And once the site gets popular there will be a lot of 6 player games.
And some people may not like 6 player games or are just tired of doing only them.
This can be easily done when the player ranking are made, you can simply ucheck all of the ranking (novice-grand master) and no1 else could join. It would be pointless to make another feature for this.