This has always been an issue with Bongo - mainly because "grouping" in Rhino is really only a selection helper. It doesn't really combine the objects. Therefore all the objects inside the group will have their own pivot and animate around it's own pivot and not work as a group.
How to solve this?
Failing that, you will need to create a proxy point, make every object in the group a child of the point, and then animate the point object.