The game-master already joined the game taking a slot, and is marked as "master".
 
There are two methods to find willing players for your multi-player-game:

1. Invite a specific (single) known player.
   
2. Add a game-offer in the waiting-room.
   

Both methods can also be used at the same time, though if you want to start a MP-game with your usual buddy, first invite him or her, set up groups, then start with adding a game-offer in the waiting-room, so the users can see against whom they have to play and in which groups.
 
Naturally you can also use only invitations, if you know all the players that should play with you or want to join your MP-game.