No pre-set groupings of resorts; it's based on how many people are going to what resorts at what times. Yes, there are common groupings, such as the monorail resorts, but if there happens to be no one going to Polynesian and Grand Floridian at the same time you are, it's not like you'll be on a bus all by yourself, but rather the Contemporary would be put on a bus with a couple of other resorts.
The maximum number of resorts per bus is 4. But if the bus has resorts from more than one geographic zone, then the maximum is 3 resorts per bus to make up for the extra travel time.
There is no pre-set order of drop-offs. The drop-off order is left up to each individual driver to decide. Again, there are common orders, such as closest to the airport first and farthest from the airport last, but it is up to the whim of each driver. On another board, one DME driver wrote about pulling out of the airport wondering which resort she would choose to drop off first.