Other than taking 'public' transportation, which I wouldn't recommend, you can take the Mears shuttle. But, if you are heading to a resort that is serviced by DME, you can't book it ahead. You have to do it when you get to MCO. It will cost about $30 r/t. If you go to the S/D, you can book it beforehand though.
As for whether or not you will be put on a DME bus....I have seen it done. But, when I was there in Sept, and taking the Mears shuttle to the Swan, they told me that only WDW owned resort guests were allowed on the DME bus, so no, those going to the S/D wouldn't be put on the DME bus. Whether or not they might put one, paying person, on the DME bus to a WDW owned resort...well, who knows.
I would imagine that the situation may be that the OP doesn't want to list someone on their reservation, hence no DME. But, be aware, that also means no EMHs, no room key, etc.