The regular theme park service begins about one hour prior to park opening (whether it's regular hours or EMH).
For park closing it's a little different. Regular service will continue for at least one hour after park closing. Depending on how many people are still in the park determines how much longer the buses run. It could very well be for another hour if the park was especially crowded.
Don't worry about being left behind. There will be at least one last bus to take the late comers back to their resorts. It will not be a dedicated bus to a specific resort but will serve as many resorts as necessary to get everyone back.
For peak periods such as park closing extra buses are put into service. I always suggest not racing out of any park at closing. Relax, walk around or sit. Maybe shop or snack and take your time exiting. Then when you get to the bus area it will be quiet again.
Disney also provides early breakfast buses. This service begins before 6:30am for the Magic Kingdom area and 7:00am for the rest of the World. If you have an early ADR just be at your resort bus stop at least 45 minutes early. This will give you extra time in case you just miss a bus.