If you want to add a player manually into your Player list in Nettla, you need to send them an invitation to join.
This invitation will automatically create a Nettla account for the player as well as add them to your club register (Player list).
Go to Players -> Player list -> Add player button, and insert the email of the player you want to add. As soon as they have accepted the intimation through the email they receive, they are added to your player list.