Zotero's new 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 as described on this page.
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.