Disney Magical Express is for any resort guest. If you're not checking into your hotel right away, can you take the DME from the airport to a different resort to begin your stay. For example, if you wanted to eat there?
However if your bus was doing a loop of other resorts , there would be nothing to stop you getting off early (or late).
They do scan bands or check paperwork again before actual boarding, so you can't just choose a bus to board.