Lewisc is correct, hugpug is not.
There are no preset routes. There are no preset groupings. At 10:00 a.m. tomorrow someone going to CBR might be on the same bus as people going to Animal Kingdom Lodge and All Star Music (which was my grouping), and at 2:00 p.m. another CBR guest could be on a bus with people going to Beach Club and Wilderness Lodge.
At least one Mears driver has posted here the general rule that for any given set of resorts on a bus, the resort closest to the airport gets dropped off first, etc.