When I create an onslaught game and build defense, if any player joins, they can't hit 'G' to start.
Once, I created the defenses solo, then 3 players joined. I hit 'G' but the rest of them couldn't and we couldn't start the game.
I had to ask 2 of them to exit and re-join to start the timer.
This happens in onslaught all the time.