This has happened to me many many times with BDC. Like 70% of the time it happens.They don't care to put the right number of guests.
What seems to work is to send them an email about 3-4 days before and say "I want to confirm your time of arrival and the number of guests so we can prepare the beds". If they are international guests then use google translate and write it in their language because otherwise it's very likely they won't pay attention to it. Also don't send the email too soon or they won't care.
If you want to avoid this situation from happening then have a different pricing for different number of beds. If more come than booked then when they arrive ask for the extra money right there before they check in. And if they raise any objections tell the to check with booking.com because your listing explains it clearly.
My advice is to allow the visits if they don't plan to have a party or stay the night.
This can even lead you to more future bookings. The friends of your guests is possible they will become your future clients. At least this has happened to me. They saw my villa and compared it to the hotel they had booked with almost double price and then booked me the next year.
As for my guests rules I don't know about that. No matter what I write and in which language they don't read them. They only pay attention to what I explain to them myself.
It is very simple why you should join. If they don't see you they won't book you. There aren't many guests who will search multiple pages back to choose their accomodation. Especially if you want early bookings.
I joined it in July and I have seen a very big boost in reservations for this and for next year. My property immediately started showing in top 5 in my area and even showed up in searches for cities near.
For my country Greece it needs a 20% commision, which now that I see that every other country has only +3% sounds very unfair? What's up with this discrimination?
Anyway it is at least much better than the Genius program, which requires a full 10% discount and also from my experience, brings guests of often questionable quality. As soon as I joined I quit the Genius program immediately and never going back.
Unfortunatly it is very difficult to stay in the preferred partner program for long if you have few rooms.