I made a Smart Group that pointed at a Group and gave it criteria of: Kind is not Group (though you could use: Kind is Any Document). This gave me a list of all the files in the Group which I could easily select and drag and drop to the top-level Group with ease.
@berndm: Just a note that this would only work in Three Pane View. I also would not be inclined to use this shortcut as it expands all Groups in the database. In a large database (ie. nested deeply) that could be problematic.
Option-clicking on the Group expands just itself and its subfolders a lá Finder.
'could be problematic’ sounds so frightening to me.
Ok, the only unwanted effect I see is, that given one has expanded some groups by default, this expands/collapses all and he would have to expand his choice of groups later on again.