You can collaborate using Zotero's "Groups" feature.  Once you have logged into Zotero, click on "Groups" from the menu bar.  You have three options from there:

1. Choose a Public, Open Membership (Anyone can view your group online and join the group instantly.)
2. Choose a Public, Closed Membership (Anyone can view your group online, but members must apply or be invited.)
3. Choose Private Membership (Only members can view your group online and must be invited to join.)

Here is what this page looks like:

For the purposes of this demonstration, the second option will be used.  You will need to name the group.  In the example below, the name "Testing 1" is used.  Once you have named your group, click "Create Group."  You will then have several options.  Here is a screenshot of the options:

Once you have selected your options, click on "Save Settings."  You have now created a group.

When you are in your "Web Library," there is a tab called "Groups."  From here, you can manage your profile, members, and library.  In order to add items to your library, click on "Group Library."  Here you can upload documents for your group.  Depending on what kind of access you have provided, these could be documents that you want people to read, or they can be documents that you wish people to edit.  This can also be a space where several people upload documents if you are working on a group research project.  The "Group Library" page looks like this:

Click on the + sign to upload a file, add an article citation, etc.

For more information and a helpful video, use this link.