Jean Grey was a founding member of the X-Men.
In the 26th century, the Earth was unfit for human life, and the Fantastic Force had transported the whole population of the Earth to Nu-World in a different timeline in the past. Due to this action, it drove Gaea mad, and caused her to travel back in time and bring four women from her Earth to force the Fantastic Force to bring some, if not all, humans back to their Earth. Jean was among these women, dubbed the Hysteries.
The Hysteries were each hand picked from different times from their Earth at their most insane and detached selves. For Jean, this could have been when she became the Dark Phoenix, as she was wearing the same uniform. After the Hysteries were defeated by the Fantastic Force, they were sent back to their proper times, Jean included.