First, set up Zotero sync and synchronize your library. To create a shared library in the Zotero app, right-click the New Group button near the top left of your Zotero window. Choose New Group from the pull-down menu.
A window will automatically pop up to the web version of Zotero prompting you to create a new group or search for existing groups.
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.