@Marcus0 Hi, I honestly think that the way we communicate does a lot to determine our professional stance towards guests.
Especially with Instant Book, I would make sure that my message after booking confirms again clearly that you are not easy to push over. I'd say something like (and I'm making up and assuming things along the way):
"Thanks for your booking for yourself, Mike. As you read in our House rules there will be a mandatory ID check upon arrival. Also note again that we do not allow unbooked/unregistered guests on the property."
(In the House rules you add "... otherwise this leads to immediate nullification of the booking")
When they write about being a group:
Hi Mike, of course it isn't a problem to bring 3 more guests. As you will have seen in my listing, there is a small per additional person cost per night resulting in 4x....... plus Airbnb fees.
I'll just change the reservation for you, and all you have to do is accept by following the notification sent to you by Airbnb.
I will also need to know their full names in advance.
However, the other 2 people in your party will not be allowed to stay in the apartment, since that is legally not allowed. They would have to find another accommodation.
If you guest hasn't responded within a few days, send them another message asking if the group changed their mind. When meeting them for check in, and the reservation is still not accepted, you simply tell them "Oh, no problem, let's just fix that and pull out your device and re-send the change. And don't let them into the accommodation until they have accepted while you're there.
Of course it's always difficult when being an off-site host, unless you have somebody in the building with an eye on everything or have a security camera on the entrance.
If it's easier for you to stand up for yourself and your prices, consider yourself the business that you are.
(PS: for my own information - There have been so many posts by hosts as well as the media saying the city of Berlin does not allow the renting of whole apartments anymore? since you are able to rent a couple of apartments, does that mean that it is still legal after all?)