As long as your carry-on luggage is of reasonable size and will fit on the bus safely without displacing people (i.e. not on seats or in aisles), there's no reason you can't bring it on the bus with you. OTOH, there's a minimal chance you may have to stow it under the bus until you get to the airport.
Speaking of which: DME is scheduled to pick up Guests at their resorts about 2.75 to 3 hours before one's flight departs. This allows time for the bus to stop at a very few other resorts if necessary and load Guests and luggage, then get all that bus's passengers to the airport with sufficient time to prevent DME from being the cause of any missed flight. So chances are you'll be there more than an hour before your flight leaves - but you still may need to check in and WILL need to go through Security and get to your gate. If you read some of the other threads here, you'll see security can take anywhere from a couple of minutes to an hour.
