Once you have selected the session type and given a name to it, a list with the team players will appear. If you want to add a player who is not on the team:
At the top of the screen to the right of "found players" the "+" button is displayed. Press the "+" button.
A new screen will open. Under the title "club players" you will find those players who are part of the club but not of the selected team.
Choose the desired player.
Press the "ADD" button.
The player has been successfully added to the session.