Zotero Groups feature allows you to collaborate with other Zotero users (must have account) via its site. Once a group is created and members invited, the Group folder appears in each member’s Zotero Library (software and web-based) on the left side. Each member may add content to the shared folder, then sync. Any new content appears in the group folder and all can see.
If you use groups, you have two sections in your Zotero collections pane (left side), and you may drag items back and forth between them at will.
How to create a group...