After you setup a group match, any player in your group is notified they can register for the match ("Opt In" or "Opt Out").

Each group has a "minimum players" setting. As players register, they will be placed in the Registrants tab until the total registrants reaches the minimum player count. Then new registrants go into the bullpen. The bullpen is emptied as a full team is created.

As an example, if you are playing a match with teams of 4, and have the minimum players set at 8, the first 8 players to register are placed in the registrants tab.

The 9th player to register goes into the bullpen. So does the 10th, and 11th. Once the 12th player registers, that group of 4 is placed in the registrants tab and the process starts all over.

The first to register are the first to play, so the only people that will be left in the bullpen after pairings would be the last people who registered.