From what I can find, the default structure for group home pages is to have one HTML box only, but you can use that single HTML box to paste in multiple widgets and HTML content. Which third-party app are you trying to add?
Max, the only provision that Ning has for reconfiguring group page elements is in the advanced settings available to the site owner (the page source, in XML format). But this method is unsupported, and even proficient coders warn against it for fear of messing up the entire site. I don't see why they didn't just include a drag-and-drop elements organizer at the group level like on personal pages.