Zotero

If you are collaborating on a project with other people you can use Zotero to create a group library. To create a new group click on the "New Library" icon and click on "New Group..."

This will take you to the Zotero website in your web browser. Login with your Zotero username and password. Once you are logged in, you can pick a name for your group and choose the membership settings:

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 "Members Settings," clicking on "Send more invitations," and entering the email addresses of your group members. 

The group should appear in your Zotero desktop application under "Group Libraries":

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.