The guests - all of them, a known set of guests staying at the hotel
Guests - some of them, maybe the ones arriving in the morning, maybe the ones passing through the lobby, maybe some other random selection of guests
In this sentence "greet guests" is better than "greet the guests". Boozer explains the difference in meaning. Since the manager does not control the guests, he can only greet the ones that go through the lobby when he is there.