A group box gets a scrollbar if there are more items in it than will fit. To force a scroll bar, create the group box too large, put your content in it, and then shrink it. I think you'll always get both horizontal and vertical scrollbars this way.
I don't believe that will help with your scenario, though, because you can't put a variable number of things in the group box to support a variable number of table items. My Creating a custom table in Personas blog did support scrolling without this, albeit a little slow. You can speed it up with the techniques in this blog - Data caching in Personas scripts - but that is getting even more extreme!
Steve.