It would be two double beds and a rollaway (yes, they now charge a nightly fee for it, didn't used to years ago).
According to the wdwinfo.com, there is an extra $25 plus tax fee for each additional adult over the two initial ones in the room, so that's an additional $75/night plus tax, plus the rollaway fee, so you're looking at $100/night extra possibly. It may be worth looking into the extra fees to determine if putting 5 in one room is even the best option. My friend did it once, it was her and her 4 young kids (ages 6-9, yes, she had 4 kids including one set of twins in a 3 year span). The room was quite tight, but as they were little kids, it worked okay for her. Not sure how comfortable most would be with 5 adults?
I'm sure you could bring an air mattress, but you still need to register the extra guest. A rollaway would likely be more comfortable to sleep on.
