Project groups (not to be confused with object groups!) are a mechanism for users to organize related sets of projects together.  For example, a user 
might have a collection of C++ projects, a collection of Java projects, and so on.  Most users won't appreciate having them all mixed together under the 
Workplaces node!  Nor will they appreciate having to laboriously close and reopen projects just to see the ones that they want.  Projects groups 
provide a solution to this.  A Projects Group is just a node, under the Workplace node, that contains a related set of projects.  In that sense, they 
function a lot like regular Filesystem groups.  By opening and closing a group node, the user can quickly and easily show and hide entire groups of 
projects at once.