When you get to the Epcot bus boarding areas, you can see there the bus's front door will be because the chained queue line leads to it. If you acctually wait in the queue with a wheelchair or ecv, you won't get seen by the dirver, like the others mentioned. And, more likely, someone else with a wheelchair or ecv will actually go in the area in front of the queue and be boarded without the bus driver evven knowing that you were waiting first.
There is an area in front of the queue that is not in the roadway. Just park your party there - sort of in the area where the back door of the bus will end up - and the driver will see you waiting.