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. Group libraries can also be used to silo citations during projects like systematic reviews, where several stages of screening occur.
To create a group library:
- Click the "New Library" icon at the top of the left-hand pane (it looks like a tiny box or cabinet) and then click "New Group."

- The Zotero web site will open in your default browser; login to your account.

- Follow the instructions to create a new group and set the privacy status.

- You'll then be asked to fine tune the settings for the group type you select.

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.