Many scorebooks may have been set to allow you to "Join". This is determined by the scorebook publisher. You can find out who this is by looking for the "Published By" section. 

You can request to join by clicking on the green join button link. Some publishers will allow anyone to join while others are more selective and may vet you before allowing you to join.  

Sometimes, you get included by the publisher without you requesting a join because they consider you to be a bona-fide member of the community that is defined by the scorebook. You can opt-out at any time.

As soon as you are included  on a scorebook, if you didn’t have one already, a account for you is automatically created.  You will be able to control this account and have complete access to it by logging in with your email address / social media profile.