RiverWare 6.4 supports only these "extended" objection selection features:

    A rectangular region of objects can be "rubber band selected" by dragging a rectangle with the mouse.  This cannot be used to extend an existing object selection because starting the mouse drag always clears the cell selection -- even if the Shift key is held.*
     
    Control-clicking (holding down the control key while clicking with the mouse) TOGGLES the clicked object in and out of the selected object set.

*Even though we're not using QGraphicsView's "rubber band" selection mode (we're managing our own QRubberBand ornament object via mouse events), this is a general problem with Qt QGraphicsView's rubberband selection mode -- SEE this article: http://asmaloney.com/2012/06/code/extending-selections-in-qgraphicsview/ ("Extending Selections In QGraphicsView") [June 2012].  (The author outlines a modification to Qt source code to provide desirable selection behavior).

RiverWare 6.5 now also provides these "extended selection" features:

    Shift-clicking an object ADDS the clicked object to the selected object set.
    Shifted "rubber band selection" ADDS the objects within the dragged rectangle to the selected object set.
    Right-clicking an object (to show its context menu) also ADDS the clicked object to the selected object set.

This third item may not seem like an obvious choice.  As is typical with graphical user interfaces, many context menu operations in RiverWare apply to the full set of selected objects.  Some operations in RiverWare were explicitly, internally being applied to both the "current object" and the set of "selected objects" -- but only the latter was highlighted on the workspace.  It makes more sense for the clicked object to become selected too.  David and I did also observe that the alternative of deselecting the other selected objects when (un-shifted) right-clicking an object was surprising.  Requiring the shift key to be held to retain the current selection -- when right-clicking to show a context menu -- is undesirable.

This last issue is the "title" topic of Gnats 5439 (1-15-2014). The use of the Shift key to _extend_ the current selection was also identified as a requirement.  Additionally, this bug report mentioned that the "Add to Object (Display-) Group" context menu operation was being applied to only the clicked operation.  It now applies to the full object selection.  But since this is a change in behavior, if more than one object is selected (including the clicked object), a confirmation dialog is presented to the user.