If you're on a paid subscription plan, you'll have the option of adding a fellow teacher to your library as a co-teacher. You can add as many co-teachers as you like.
First, the teacher will need to join your library using the code you give them. Then, you will be able to use the Author picker dropdown (found on the left of toolbar, underneath the library name) to promote that teacher to a co-teacher. Use the 3 dot menu next to the person you want to promote.
Once you've done that, the co-teacher will have all the same admin rights to manage the library settings, including the ability to invite new students, edit and publish books, and turn on real-time collaboration.
Can I promote students to co-teachers?
No, this only works for users signed in as teachers. Students will be listed as 'AUTHORS' in the author picker and you won't see the option to promote them to co-teacher.
If you're having trouble adding a colleague as a co-teacher, it's worth checking whether or not they have signed in as a student by mistake. Get them to sign out and sign back in again as a teacher before you try again.
Also note that you cannot add co-teachers to your private My Books library, as this can only be viewed by you.