There are 2 ways to create a group with Zotero:
New Library...icon and choose
New Group...(this will take you to Zotero.org)
Groupspage, then choose
Create New Group
Groups can have a variety of privacy settings, and enable you to share references with a select group of people or the world.
In order to use groups within Zotero, data syncing must be enabled. When syncing is enabled, group libraries will appear in the left pane and can be used in exactly the same way as the personal library.
Groups may use file storage to store and share files among members... however, this counts against the group owner's 300 MB file storage limit. Zotero's recommended workaround for this is to create a separate account for the group leader role.
To add members to a group:
Groupsand create a new group if you do not have one already
Send More Invitations
For more help using groups, see Zotero's very thorough documentation.