This is horrible and I am sorry you are dealing with this.
While your question seems simple, it is not.
Do you charge per person? How did you find out? Did you say anything to the guests in the moment?
The short answer is that you can file a claim. The long answer is that you need to be thoughtful about how you are doing that,
it sounds like you are on the property. Did you immediately ask the guests to leave and was it done through Airbnb? Unfortunately Airbnb will site with a guest unless you have solid proof and good policies in place.
I use a device called party squasher to keep track and document extra guests. I also ask for an ID from all guests and their visitors to be sent upfront. These tools are not magic but they have both really helped to cut down on extra visitors, damage, irresponsible behavior, etc
Back to the claim. You begin boy requesting money from the guest and if they decline, you involve airbnb. Good luck!