Sadly there are guests that try and take advantage of new hosts by bending and breaking the rules. @Elizabeth1894
In this situation, you have two choices, you need to go there and talk to the person who booked...you can;
a) ask the additional guests who have not booked and paid to leave immediately AND tell the guest who booked if anyone else comes who hasn't booked you will ask Airbnb to cancel the book and they will all need to leave
OR
b) tell the guest you are disappointed they bought guests into your listing who have not booked and paid. Tell the original guest how many of the additional guests you are happy for them to have stay (once they have added them to the booking and paid for them).
Now the booking has started you can't do a booking amendment for the additional guests, so call Airbnb ask them to add the additional guests to the booking; your guest will then need to accept the request for the additional guests to be added for the extra guest charges to be added.
It sounds like it would be worth you familiarising yourself with the Q&As in Airbnb Help which cover the basics around managing your Airbnb to help avoid these situations in the future.
Do make sure your house rules make it clear only those who have booked and paid can visit or stay.
I always vet my guests before accepting their booking, one of the questions I ask them to confirm is a) who is accompanying you on this booking?
I would recommend as a new host you remove Instant Book until you are more comfortable with vetting guests and know what signs to look out for in terms of red flags. So if you have a three bed place for example why would only three people book to stay.