Add and arrange references into folders ("Collections"). A reference can belong to several collections at once, and adding it to multiple collections doesn't create copies of it.
Create shared folders ("Groups"), and share references easily with other Zotero users.
You can add and arrange references (items) in collections. Items can belong to multiple collections and sub-collections. Adding an item to multiple collections does not duplicate the item. In this sense, collections are more like music playlists than folders in your computer filesystem. Just as a single song can be added to more than one playlist, a single item in a Zotero library can be added to multiple collections.
The library root, "My Library”, always shows all items in the library, and items are duplicates only if they appear multiple times in that view.
See more information about collections from Zotero's web pages.
Image: Collections in Zotero.
With Groups in Zotero you can share references and collections with other users.
Group Libraries are separate from your personal My Library, so you can also use Zotero groups as a way to keep separate libraries within one Zotero profile.
Create a new Group in Zotero (see image below) or directly on Zotero's web page.
See more information and instructions on creating and managing Groups from Zotero's documentation..
Image: Group menu in Zotero.