Is there any way you could rig up an animated. This is how you play thing?
Maybe mention a few times. "Some of the games will take a long time between turns as there is 24 hours available to each person. Don't dispair check back once a day and it'll go smooth most of the time."
Or make it so the first game a person joins is all first time players. Once they get through that first game people usually get the idea and don't just dead beat. Right?