I'm simply assuming that the who is the people who lived in this area. It appears the inhabitants here were a motley assortment of those who migrateds in from the west as the desert dried (Laird Scranton has some interesting thoughts here) and peoples from the south. It appears Egypt always had a lot immigrants as well but the culture came from the west (and south). These people lived on the "horizon" above the valley and high up onm the valley sides. They occupied all the oases which would have been more numerous in those days. Giza was an extremely important site to them economically and militarily.
Perhaps these people got tied up in the Atlantis myth before or after they arrived. Language is confused and no records exist. There are a few tantalyzing clues that there were different peoples in the area. The Pyramid Texts is the oldest surviving corpus in the world and much of the PT was very ancient when it was "first" recorded" in stone. One line is;
423a. To say: Lo, this foreign country of the mouth of the river, this is thy complaint:
423b. "This foreign country of the mouth of the river belongs to me, the lord of Ḥknw."
423c. It is Ḫ‘i-tȝw of Ḥknw, this thine ox-god, the renowned, against whom this has been done.
If you believe in ancient religion (I don't) then you can also imagine that Giza was a religious site and the Great Pyramid a religious icon built for stinky footed reasons as far back as 5000 BC just as some carbon dating suggests.
What the hell is meant by "who" were the people. Everyone's different and nobody is representative of a "culture". Our heritage, language, and genetics are all different. We simply don't know anything about the people who lived there at that time. They could have all sat around watching aliens build it for all I know.