Jean Paul Sartre is an existential philosopher from France. Sartre as an existentialist has strong beliefs in free will/responsibility for choices you make, and individualism. Sartre does not believe in a transcendent force or a god, he believes that people make choices in their lives and those choices are 100% made by free will. With this idea Sartre also believes that people are also fully responsible for the decisions they make in their life. This theory goes for every single decision that a person makes, no exceptions it was your choice. Another important Sartrean theory is, his theory of individualism. Individualism is exactly what the name entails, Sartre believed that people should rely on themselves and stay individual, outside of a group. But this does not mean that everyone should be anti-social, Sartre himself was a pop icon. Sartre believed in this so much that he did not accept the Nobel Prize in Literature because, it would mean he would be part of that group. He knew that his work was good and he didn't need anybody to give him a prize to know so. Sartre's ideas were looked upon as a popular belief to some during the 60's and 70's.
When the thought of free will and humans is brought up, Sartre would say that every single action and decision is made purely by free will. With no transcendent force "fating" what you do humans are left to make decisions that make who they are. Humans have no pre-defined purpose or future, by free will people make who they are. This goes with his belief of individualism, society or other forces don't make who you are decisions that you make, make who you are.
With your free willed decisions that you make there is no one else, but yourself, that can take the responsibility for your actions. Sartre sees responsibility as something strong, because we have 100% free will we also have 100% responsibility for our choices. Although Sartre believes in individualism, this does not mean that actions that you make do not affect the other people in the world which is why we still need to take responsibility for them. So According to Sartrean Existentialism humans have all the free will they could ever take responsibility for.