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, they click on the 3 line menu in the far left of the toolbar in the toolbar, and they'll see the link to Join with a code. This applies to teachers too, but instead they'll open up their Teacher Dashboard.
How to leave a library
If you've joined someone else's library, you can leave it by clicking the Settings cog in the toolbar.
You'll see this: