From Competitions, select Team Setup.
Click on the name of your team.
Click on the Fixtures tab. This will list all the games your team is playing.
Click the Confirm Members button next to the game to which you need to add players.
This will give you a list of available members on the left. Tick the box next to the members you want to play in this game. Click Add to Fixture.
The names will move to the list on the right. Click Save.
NOTE: when adding members, you can include the coach and team manager.
NOTE: the Add to Fixture button will only appear once you have ticked a box.