Hi @Leslie7
I already looked for this feature and couldn't find it...
I wish we could add a "personalized" fee.. A lot of hosts have extras to include on their houses.
The only automated way is using the daily rate for extra guests...
The other option is not automated and a bit "out-of-the-way" to do it... You always can use the resolution center to charge additional money from your guest.. Please make it VERY clear on your description that if they have more than X guests you will send a additional charge via the resolution center...
If you make it VERY clear on your description you may also collect the payment in cash from the guests in case they bring that extra friend... Airbnb does not recommend collecting extras fees in cash (they can't guarantee you will be paid) but they also let you do it IF you make it VERY clear on the description...
Luis