Having a group library with others is great when collaborating. You can also join existing public groups, or groups that are closed or private to which you've been invited.
To create a group library:
From the Zotero client, you'll see your group (or groups) below your own library in the left-hand pane.
You can drag and drop items between libraries; this only copies a reference, rather than moving it.
IMPORTANT NOTE: Your personal and group libraries are entirely separate entities. Making changes to an item in your library (for instance, adding information to the reference, or attaching a full-text PDF) will not impact a group library you belong to, even if the item is in both libraries.
When you'd like to invite members to join a group you started, follow these steps: