A family of Microsoft relational database management systems designed for ease of use.
Adding to Julie's excellent response, I believe Project doesn't compute the "complete-through" date for group summaries because, unlike summary tasks, they do not have an assigned or inherited calendar. You can add a custom finish field to approximate the complete-through date using a specific named calendar, then use that field as the endpoint of your group summary progress bar. Here's a simple formula for the new field using the "Standard" calendar (i.e. the typical project default calendar):
ProjDateAdd([Start],[Actual Duration],"Standard")
You'll have to modify the formula if your project calendar is other than "Standard." I've assigned this formula to the [Finish2] field. When specifying the custom field, be sure to check the "Use formula" radio button for "Calculation for task and group summary rows."