Would it be possible to make the fee for extra guests adjustable according to demand, season, weekday, etc. Large groups can generate more than your base rate in extra guest fees, but the extra guest rate is fixed. If I'm in low season and I try to encourage large groups by cutting my extra guest rate, I could be losing money on advance bookings in the higher season.
I guess this would require some integration with AirBnb because they only have one setting for extra guest fee.
Another option is to provide recommendation on guest fee.