I started a nine player game with some friends. One player declined. The game says that slot is now "open." Does a new player have to fill that slot for the game to start? Or is there a way to launch the game now with eight players? It feels as though there should be that option, but I cannot find it lol. Thanks in advance for any help!

As they join, all players agree to play a specific scenario of the game with certain time constraints, amount of fog and number of players. To change one of those parameters would be unfair to players who joined expressly because of those conditions. If ALL players agree to change any or all of those parameters, simply delete the game and start another.

I agree with you that it could be an option, but it is not. You need to make a new game with 8 or find a new friend to fill the last slot.

If you do want to just fill the slot, you can skim through the forums and invite a few of the regulars. Most will be happy to play almost any map to fill a slot for you, and if they don't want to play they'll just decline.