@Debbie889 You're under no obligation to accept these guests, and if I were in your shoes I would have refused them outright. Airbnb bookings are not transferable - whether it's prior to the scheduled stay or midway through, the contract between you and the guests you approved does not extent to whatever people they decide to hand your home over to. I would have advised the guests that in order to accommodate their friends, they would first need to cancel the remainder of their booking, opening up the calendar for the friends to request a separate booking at a discounted rate.
Where do you stand if something goes wrong? Well, the original guest is still responsible for any damage, and they'll be the sole recipients of your review. But if you were to file a claim that they disputed, you would likely lose because the Host Guarantee is technically invalidated by the presence of unregistered guests.