@Janetta3
1. Go to the page with your calendar
2. Click on 'price settings' on the vertical bar along the top of the page
3. In 'pricing' look for the option 'extra charges' and press edit
4. At the 'extra guests' box you can enter a price and at ' for each guest after ' enter a number . eg After more than: 1 guest, charge 300 per person, per night.
Ok. you have made the pricing error already so any changes will only be made to future bookings.
With the booking that has been made you can send a message to the guests and explain there was a mistake with the price and hopefully they will compensate you.
Otherwise you can either take one on the chin and learn from your error - or contact (https://www.airbnb.com/help/contact_us ) airbnb and see what they suggest to do - cancellation is also a consideration but it can incur varying penalties, read up on the rules beforehand.