If a Group has a Placeholder, then the placeholder’s size is determined by the area covered by the group’s member nodes and links.
So it would be quite natural to have the group’s width be dependent on the width of the member nodes if the group layout arranges them in a single column. Most group templates have this feature.
But if you want resizing a group to change the width of its member nodes, then you’ll need to customize the ResizingTool.