I've had a few guests bring overnight guests. It worked out fine.
One actually booked the room for 2 people even though he was travelling alone. I asked him about it, he said, he just wants to make sure the reservation has the right count in case he has a guest over.
It seemed odd, yet smart at the same time. For those who charge an extra person fee for the 2nd person, this covers the cost.
but it is a bit odd in a shared home to have someone "bringing a trick" over for the night.
Ultimately the guest is responsible for their guest, but you don't know this persons critera for choosing people and if someone sneaks off in the middle of the night with someting of yours it'll be inconvenient.
for these reasons, many hosts forbid any guests that aren't on the reservation.
Others don't really care as long as it's not a party going on.
I think you just have to decide what's best for you, but for liability reasons, it's probably a good idea for the guest count to be up to date in the system.. if someone books for 1 and there are 2 people it's unclear whose responsible for them.. whereas if the guest books for 2 and there are 2 people you're likely covered for anything their guest does in that case.