It's a difficult lever to move, isn't it? I've always done it as a guest, ever since I began using Airbnb; it just seems like good manners to me.
Since beginning hosting, it's apparent that not everybody (very few, in fact) provides the details of their travelling companions. We're using Scheduled messages to provide info to our guests as their stay approaches (and during), and I've added a prompt there during our booking acknowledgement message to see if that makes a difference.
It's not a huge deal, but I would prefer to know who's coming with the booking guest. There's also the added benefit that. the additional guest(s) get the reviews on their profiles as well.
Steve