Once you've created a library and want to start inviting students, the easiest way is to click on the Add students to library button that you'll see in the centre of the screen, just below the toolbar.
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Here's the whole process explained for inviting students to join your library.

If you don't see the link to show the invite code at the top of the library, this might be because you are not the owner of the library. Perhaps you are in someone else's library? Go to the Teacher Dashboard and check if you are the owner or if it's a library you joined. 

If you become a co-teacher of a library, you will then be able to share the invite code.

It might also be because you're in My Books. My Books is your private library just for you, move your books to another library and invite users to that library to share them.