The doors are opened by: a witch. You would know she was a witch if you saw her in the grocery store. You'd strongly suspect it even if she was wearing jeans and a t-shirt, if it was physically possible for her to be wearing anything other than green-black robes and a pointy hat, which it isn't.
She looks them over with eyes that have seen everything. She has been here before them and she will be here after them; she is as inevitable as gravity; she has never been surprised and she will never be bored.
"Thank you, Hagrid. I'll take them from here." Hagrid holds the door as McGonagall glides away, leading them through a magnificent entrance hall that must be three stories high if it isn't higher. Across from them is a second, even grander set of wooden doors, bound in iron and carved with swirling vines. She turns aside from these, toward a smaller door, and leads them into a little annex off what must be a truly massive hall. Through the opposite door they can hear the low murmur of a hundred conversations.
"Welcome to Hogwarts," begins the speech, echoing softly off walls that have heard it so many times they could probably give it themselves. "The start-of-term banquet will begin shortly, but before you take your seats in the Great Hall, you will be sorted into your houses. The Sorting is a very important ceremony because, while you are here, your house will be something like your family within Hogwarts. You will have classes with the rest of your house, sleep in your house dormitory, and spend free time in your house common room.
The four houses are called Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. Each house has its own noble history and each has produced outstanding witches and wizards. While you are at Hogwarts, your triumphs will earn your house points, while any rule-breaking will lose house points. At the end of the year, the house with the most points is awarded the house cup, a great honor. I hope each of you will be a credit to whichever house becomes yours.
"The Sorting Ceremony will take place in a few minutes in front of the rest of the school. I suggest you all smarten yourselves up as much as you can while you are waiting. I shall return when we are ready for you. Please wait quietly."
She slips through the door into the Great Hall, leaving them all alone for a minute.