Creating groups enables you to share references with others. The first step is to create an account. Go to File → New Library → New Group. This will open your default web browser to the Zotero login page. After entering your Zotero account credentials, you will be redirected to the Create a New Group page. Your group can be either private or public. Click Create Group to finish.
If your Zotero account is synced, the new group library will automatically appear in the left-hand panel under Group Libraries.
If all members cannot see all items in the group library
If some items in a shared library are not visible to all members of a group, this may depend on the owner of the group not having enough storage capacity. The owner can solve this by either stopping the synchronisation of his or her personal library, and then removing some of the attachments in the online library, or by buying additional storage space.