If you are the team leader then you can add new players by clicking in League-Systems.com and go to your team. Then click Add member in the "Team members" box.
If you do not have a team leader in the team, you need to contact the league's administrator who can add the new player, or make one of you a team leader.
As team leader you can change the player's role. You can also remove players and hand over the leader role to a new team leader.