Once a library has been created, students (and other teachers) can join it and add books.
(Note, if students are using login links to sign into Book Creator, they won't need to input the library invite code).
If a student is already in Book Creator, and 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.
See also: How to join another teacher's library