I know this post is a few months old, but I've been thinking about the same thing. There are 8 of us, so we could fit in a 2 BD, but I'd like to give some folks their privacy. I'm planning: my mom and sister in the studio; my niece and boyfriend on the pullout couch; my hubby, me and 2 school age kids in the master. I just hope that's enough privacy for my niece and boyfriend - I've got to see what she thinks.
You could put the air mattress next to the master bed. Or put it in the master bathroom if you'd like more privacy.
Or in the living room if that works for your situation.