Once a library has been created, students (and other teachers) can join it and add books.
Students will be prompted to enter the library code as soon as they first login to Book Creator:
If a student wants to join a new library, click on the Libraries button in the toolbar, and you'll see the link to Join with a code. This applies to teachers too.
How to leave a library
If you've joined someone else's library, you can leave it by clicking the Libraries button > 3 dots > Leave library.