Ok, i see. The support for groups in Forest is specially designed to create clumps of plants. Once scattered the items are handled as individual elements, using their own pivot and collision properties. So, probably it's not the best method to be used here.
If you want to handle each car (or tree) as an unique element, the suggested procedure is attach all parts in an unique mesh. Since Forest Color is used at texture level, should not be problems to create one or more color variations in the multi-sub material, generated from the attaching process.