In a formal, commercial setting (such as a restaurant), you could use Maitre d':
maître d'hôtel (also maître d')
1 The head waiter of a restaurant.
1.1 The manager of a hotel.
Another option, perhaps, (again, depending on the situation), could be a Concierge:
1 (especially in France) a resident caretaker of a block of flats or a
2 A hotel employee whose job is to assist guests by booking tours,
making theatre and restaurant reservations, etc.
Lastly, (and this is a bit of a stretch,) there is Attendant:
1 A person employed to provide a service to the public in a particular
place. ‘a cloakroom attendant’
1.1 An assistant to an important person; a servant or courtier. ‘a mosaic of the Empress Theodora with her attendants’
2 A person who is present on a particular occasion. ‘he had become a
regular attendant at chapel’
FWIW, the folks that say "hi" at the door of places like Wal-Mart are called greeters.
Hope this helps!