Gaetano and Paolo are great hosts. They replied to messages quickly and concretely, contact was hassle-free. We got a great list of recommendations of what and where to eat and what to see in Naples.
I had a total of 9 people in the apartment, I made an appointment with Gaeatno at check-in, but apparently there is also an option of self-accommodation.
The apartment is huge - it is probably two apartments connected and 12 people can easily fit there - two kitchens, two bathrooms, 4 separate bedrooms and a few extra beds. In the case of larger groups of people, it is worth paying attention to louder behavior at night - you have to respect the other inhabitants of the building.
Flat were well equipped also, nice, in a good location in a fairly young building. 15-20 minutes on foot to the station, less than 10 minutes to the big Carrefour.
I highly recommend these great hosts and this apartment for large groups and everyone else!