I believe that feature roughly exists in the concept of galleries. Each installation of the web application is associated with a particular gallery ID (default is "1"). Each gallery can share a common database and set of users, but is independently managed.
For example, one can set up one gallery at
http://www.yoursite.com/robert/ and another at
http://www.yoursite.com/susan/. Each has a different gallery ID but uses the same back-end database, including users. It is also possible to use the same database but have a different set of users for each gallery.
See the sections
Working with Galleries and
Membership Configuration in the
Administrator's Guide for more info.
Roger Martin
Creator and Lead Developer of Gallery Server Pro