From TDN

I clicked "Discussion" on the "WorldBuilding/MissionEditor/SimGroups" page... hope that was the right thing to do to add my feedback.

The process for adding an object to SimGroups also allows dragging and dropping of objects into SimGroups. I don't know how robust this is (haven't had a chance to play with it much) but it's a nice addition to the old method of hitting "Add selection to Instant Group" that's described in the docs.