Zotero's Groups feature allows you to share references with other Zotero users online. It's a great way to work on collaborative research projects.
First, set up Zotero sync (see tab above).
Next, log in at Zotero.org. There's a "Log In" link in the top right corner of the page.
You'll now have two sections in your Zotero collections pane: My Library and Group Libraries. You can drag items back and forth between them at will.
Zotero groups uses your Zotero File Storage quota.
To get the most out of this space, set up Library Lookup in the advanced preferences.