Sharing individual notes, or whole notebooks is very useful - and there are a number of ways you can do it. Here's how:
Click on this icon when you see it on a note, notebook, or once you click on the symbol on a note or notebook card.
You will then see this screen:
To share the note or notebook with people, start typing their name in the 'Add People' section.
You can then select whether you want the person or people (add as many people as you like) to be able to edit or just view the note or notebook (both will be selected by default). Then make sure you click the 'ADD' button.
To share your note or notebook with a whole group - select 'Groups'