On the Roster page you will see a red button under Manage Players, that says "Add Player." If you click on this, it will ask you to put in the team member's email and select their role (you can add them as player or coach). Your team member will then get an email from Scoreboard asking them to click on a link and pick a password, which will activate them as a Player on your account. They should NOT try to sign up for their own account prior to receiving the Confirmation email you send them. 

Here is a quick tutorial video demonstrating the process.