In his famous lecture “Existentialism Is a Humanism,” Sartre defines existentialism:

What do we mean by saying that existence precedes essence? We mean that man first of all exists, encounters himself, surges up in the world—and defines himself afterwards. If man as the existentialist sees him is not definable, it is because to begin with he is nothing. He will not be anything until later, and then he will be what he makes of himself. Thus, there is no human nature, because there is no God to have a conception of it. Man simply is. Not that he is simply what he conceives himself to be, but he is what he wills, and as he conceives himself after already existing—as he wills to be after that leap towards existence. Man is nothing else but that which he makes of himself (Sartre, Jean-Paul. Existentialism Is a Humanism, translated by Carol Macomber, Yale University Press, 2007).

This is Sartre’s “first principle” of existentialism. For the existentialist, creatures and objects simply exist. Deciding that we, collectively, are humans is an essence that we place on our existence afterward. We are each subjective creatures that exist in the world individually first and as a collective group of humans second.