This looks like a bug, looking at the running 4P+ games only showed games with 4 players. I'll forward this.
My suggestion: Do not use the table view to join such games, instead use the automatch screen (right side, friends pane). Declare each other as friends in the game (in the friend list), you create the game, and everybody that declared you as a friened will see that game in their automatch screen. Then everybody can join for a 6 player game.
That also works if a friend of a friend wants to join. Your friend sees your table with the game you created. Once they join, their friend sees them (at your table) and can also join.