Step 1 - Turn on Duties
The coach / manager needs to have duties settings allowing team members to volunteer. You can see these settings below - in particular with the top setting which lets the team choose their own duties.
Step 2 - Setup Your Duties
So there's a few ways to do this - you might add them to games individually but by far the best way would be to setup a duty template.
Step 3 - Ask for volunteers!
Send out a note asking for your members to signup. I'd do this by email - use the magic team email to make this super easy. To make people's lives easier - you might want to also include come helpful information such as how many duties they need to do in a season - and where they can signup.
Step 4 - Signup Time
Your players and parents can sign up themselves, and they can do it on mobile or on web. It's too easy so there's no excuses for not being part of the team.