This may sound biased but I think that it would be nice to be able to size the group bounding box. Use the union of the inner bounding boxes as the default bounding box for the group, but allow the programmer to resize it by code.
Thinking it again, I believe that the group should be just like a rectangleNode with the extra property of a list that holds the inner objects. The group should be paint before the inner objects and should get priority on the mouse clicks. This approach allows the group to also have a label, its own color and/or background image, margins, etc. etc. It would be also easy to drag'n'drop nodes inside the group.
I guess you agree with the functionality but I am not sure on the coding requirements to implement it.
What do you think?
EDIT:
What about adding groups inside a group? Maybe you should consider the current graphNode as the groupNode (that is, provide it with the ability to hold possible clild objects, even other groups). Then.....
Well, it's bloody complex
. We need well defined specs before putting the hands on it.