Word for a person who meets all the guests at the entrance

In a formal, commercial setting (such as a restaurant), you could use Maitre d':

maître d'hôtel (also maître d')

NOUN

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:

concierge

NOUN

1 (especially in France) a resident caretaker of a block of flats or a small hotel.

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:

attendant

NOUN

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!

You should consider usher.

ODO:

usher NOUN

1 A person who shows people to their seats, especially in a theater or at a wedding.

‘Sarah and Paul have asked me to be one of the two ushers at their wedding.’

American Heritage Dictionary:

usher
n.

2. One who greets guests at a wedding and escorts them to their seats.

In the PTA world, this has been traditionally called the

Hospitality Chair

or

Hospitality Committee

It's okay if it's a committee of one. It is sometimes called the

Welcoming Committee

Again, it's okay to have only one person in this committee.