Zotero allows collaboration between multiple users which is great for group projects. Groups and Group Libraries need to be managed through your free Zotero Web Account and are synced across multiple group members and devices. Your group library operates the same as your main Zotero library, however, files added to a Group Library will not automatically upload to your personal Library.
Ensure your online Zotero account is set-up
Go to your Web Library then select "Groups" from the top tool ribbon
Select "Create a New Group" (see green box in image to right), enter a name, select a membership type (recommend private), and click "Create Group"
Invite members to your Group - all members must have a Zotero web account to enable syncing across members' devices