The group editor is the icon at the top of the edit screen at the far right end of the row... with what looks like a dotted box in it. Be carefull using it though as you can do more damage than good if you don't watch what you do. The grouping tool is imprecise and a pain to use, but it is extremely useful if you want to re-assign a different texture to an item or part of an item or group of items. Anyway, the best way to figure it out is to play with it, but be careful. :)