Sharing a OneNote Notebook, Section, or Page is done either through OneNote or OneDrive
From OneNote:
File > Share> Invite People > Enter the name or email address of the person you’d like to share the content with.
From OneDrive Web App:
Select the option under the ‘Sharing’ column of the selected Notebook > Invite people > Enter the name or email address of the person you’d like to share the content with.
