@Hazal-Isinsu0
Firstly, your wording is confusing, It seems you are a Host who will receive guests who wish to bring an extra person.
In any case, either party has option to make this change.
On the booking page, below on the left side you see in blue CHANGE or CANCEL.
- Click on CHANGE.
You can then change various details including Number of guests.
If there is extra charge for additional guest (as in this case), the sum will automatically revise to the set listing price for the extra guest.