Groups in Visio have well defined margins independent on group content (see picture below). Is it possible to visualize this margin, i.e. to assign line color and width to it?
I'm not aware of any automated tool for doing this. In fact, in Visio 2003, there is a tool in the Shape->Operations drop down, to update the alignment box, to re-set the width and height of the group shape to match the bounds of the sub shapes.
This is easy to do manually if you are acquainted with modifying geometry in the shapesheet. You would just insert a geometry section in the parent group shape, and have it draw a box using the width and height of the group. This would also be easy enough to automate in VBA, as well.