The character you play in Portal is never named throughout the game. I can see that it's a female by looking through a portal, but the game always skirts any information about the character (e.g. GlaDOS speaks to you, but cuts out the name by saying "Subject name here.") I understand that Valve did this because they want you, personally to become more immersed in the game, but I think having a compelling character with a great story makes a good game even better.
Although Chell's origins are unknown, she was most definitely among the people present during GLaDOS's activation in 200-, as GLaDOS locked down the facility after her activation, letting none escape, or be let in from the outside.
EDIT: Visiting aperturescience.com, you can learn that she is the daughter of Cave Johnson (CEO of Aperture Science) and that she was brought there on Bring Your Daughter To Work day.