As long as the event you're attending has created and enabled the guest player pool, coaches and managers can contact any registered guest players to their teams with these steps:
1. Log in to your account, click on team management, and select the desired team to add a guest player.
2. Click on Team Registrations and select the event you would like to add a guest player for.
3. Select Athlete Pool and Click on send a message
4. Complete the message and press send