In design mode, users can use the AnnSelectionObject to select one or more objects. Calling Group will group all of the objects in the SelectedObjects property into an AnnGroupObject. From that point on, all of the objects in the group are acted on as one object. If you select one object, all objects in the group become selected. If you select the group and edit it, you are editing all objects in the group. For instance, if you change the Fill property for the AnnGroupObject, it will enumerate all objects in the group and set the Fill property for each object that has its SupportsFill property set to true.
Once you have "grouped" a set of objects, you must ungroup them before you can once again select a single object from that group. When you select a "grouped" set of objects and call UnGroup, the objects will be ungrouped. After that, the objects are independent again (you can select one without selecting the others). Note, when you select one or more objects and group them using Group, an AnnGroupObject will be created and the selected objects are moved from the AnnContainer to this Fill property for the AnnGroupObject. This AnnGroupObject is a temporary group and will be ungrouped (by calling Ungroup) and added back to the AnnContainer when the objects are no longer selected.