@Brandi3 Go to your "Reservations", find your current one, and then select from the menu on the right the choice "Change or Cancel".
Then, select "Change Reservation", and you will see now a form where you can edit the number of the guests.
You also can see the section "Subtotal" which refers to the total price the guest paid you.
So, you can add the two guests at the number of guests and make it 4. You say that the additional guests came half way through. So, you can estimate how many days the extra guests stayed or will stay and find which the extra amount that you would like to ask from your guests. I mean, they were not 4 from the first day, so maybe you can adjust the price accordingly.
Of course, that was a not a good action from their part, having two more people without telling you. But anyway, as hosts, we see many times action like that. So, you can communicate with them and explain why you are making that change.
Of course, if they refuse to accept the change, contact Airbnb!
I wish you the best!
Ira
For anyone who wants to contact Airbnb, see this thread:
Let's all remember to give thumbs up to hosts that help us!