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 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.
Quick tip: Be sure to setup Group settings This is where you name your group, invite group members, and determine access rights for your group members. On the Library Tab for your group, be sure to give your group members rights to add and edit (if that's appropriate).