Zotero's Groups feature allows you to share references with other Zotero users online.
Make sure you have registered your Zotero account, and synchronised your library.
To create a shared library: click the Create Groups button (top left of your Zotero window) 
Log in to the zotero.org website to create or join a group.
To join an existing Zotero library, search for it at zotero.org/groups or be invited by the group’s owner.
You now have two sections in your Zotero collections pane: My Library and Group Libraries.
Personal and group libraries are entirely separate, and changes made to items in one library do not affect the other. You can drag items back and forth libraries to copy items.