If you're set on DVC rooms (and not simply deluxe hotel rooms), I'd consider renting DVC points and looking for connecting (or adjoining) studios or a 2-bedroom suite. We visited with a party of 5 and stayed at Wilderness Lodge in a 2-bedroom villa: on rented points, we saved 40% off of the rack rate for the villa. ("Rack rate" meaning, what we'd have paid if we booked it directly through Disney.) The downside of rented points (which you should only rent through a reputable broker or from a DVC member you know and trust) is the fact that you can't apply other seasonal discounts, you need to plan far ahead, and in many cases you'll want to purchase vacation insurance as it's nonrefundable in the event of cancellation.
Otherwise the most economical choice would be getting connecting (or adjoining, depending on preference) standard rooms at one of the deluxe DVC resorts, rather than DVC accommodations, as most of the DVC rooms (studios everywhere except OKW, 1-bedrooms) have foldout sofas as half the sleeping spaces instead of proper beds.