If you are collaborating on a project with other people you can use Zotero to create a group library. To create a group library, you can either click on the File menu, select New Library, and then New Group... or go directly to the Zotero Groups page on the Zotero website. Login with your Zotero username and password. Once you are logged in, you can click on "Create a New Group" and choose the membership settings. You can create a public, open membership group; a public, closed Membership group; or a private membership group.
Keep in mind that open public groups do not allow file sharing. If you want to share PDF attachments with others, you should create a closed public group or a private group. Any file storage will count towards the storage space of the person creating the group library.
After creating the group, you can invite members by going to "Group Settings" and then "Members Settings." (If you are viewing all your groups, you can click on "Manage members" to be taken straight to the members settings page). Click on "Send more invitations," and enter the email addresses of your group members.
The group should appear in your Zotero desktop application under "Group Libraries." You may need to click the sync icon to get it to appear.
You can drag any items from your personal Zotero library or add items directly to the group library to share them with your group members. You will need to click the sync icon in order for all group members to see added items.