@Yadira22
It seems you are getting plenty of warnings, but not doing anything to counter it, failing to uphold your own rule: 'no guests outside of the registered group.'
Firstly, it's extremely rare for 1 person to book an entire flat for 6 people.
For those 'informing me via the Airbnb messaging service', you simply reply via the messaging service that numbers need to be precise, adding that Airbnb 'Host Guarantee' won't apply, therefore you can only accept the booking if a formal ALTERATION to the booking is applied.
For those who don't inform you, as soon as you discover it, you phone Airbnb to contact the guest with ultimatum to pay the difference or booking will be cancelled.
I would only acknowledge you face a greater difficulty as you have self-check in. Perhaps also if you use Instant Book? (I don't know how it works since I don't use IB.)
The only alternative would seem to be to scrap self check-in and be there in person to greet every group.
Finally, just to clarify, as a live-in Host, and one who doesn't do IB, though I'm occasionally confronted by this issue, it's very simple to resolve. It's more common that Enquiry is for 1 guest but a plurality 'we' is mentioned in the message. I reply that 'Airbnb will only process the booking & apply Host Guarantee with the correct number of people. --' It's far more rare for the single guest to book only to inform me afterwards and still prior to arrival. Anytime it's happened I've sent the guest an Alteration request with which they've complied.