I started hostingjsut recently and I had the same problem already after the first few hostings. So I started double checking with my guests how many people will be visiting after every booking. Some of them admitted that they will be bringing more people than indicated. To those who admitted, I've sent the booking alteration request. All except one agreed and send paied extra guest fee.
However, I noticed that these people then tend to subtract a star for the value element. Out of 6 such cases, 4 guests rated lower for the value, even though I had reduced prices to collect reviews. There is a connection for sure, because out of my 18 reviews only these 4 guest gave lower stars for the value, whilst all the remainig guests gave me 5 stars in all components.
So, now I decided that I won't ask for alteration anymore (at least for the initial period). I will instead report this behaviour to Airbnb. Hope they'll act on this.