After all players have joined a multi-player-game you as the game-master follow the link "Choose order" on the "game-players"-page to set the order in which the players in each team plays.
 
Each team has its own set of playing-order starting at 1. Use the select-boxes in the "Set Order"-column to choose the order in which you want the players to move within each group. If you are satisfied with the order, save it by clicking on "Save selected settings". If you used an invalid choice for the order, the orders are not saved and an error will tell you what you have to correct first.
 
You might want to think strategically about the playing-order taking into considerations if stronger players have to play before or after weaker or stronger players. But as multi-player-games are for fun, don't overthink it and choose a fair setup!