Hi ScrapYap, I think this may have changed:
"Can I add guests to my existing resort reservation through my MDE?
No, you would have to call guest services to add guests to your existing resort reservation."
I was able to add three new people to my room only reservation online by clicking on "Change Reservation" under MDE (through the website, not sure about the app!). I originally made the reservation back in July.
I've got my fingers crossed for the SDFP loophole to remain open until our November trip!