my friend, who's 17 left her house [for reasons unknown] and now lives with us. my parents are going to take responsibility for her. what does that entail?

Whoa. What do her parents say? Do you know your parents could be accused of kidnapping?

You need to call in your local Family Services to deal with this. If your parents want to act as foster parents to this girl, they have to get the approval of Family Services.

No they can't just "do it" to actually have any responsibility at all, the friends mother and father needs to sign a power of attorney at a minimum to allow them to even sign or do anything for them at all.

Next to have actual responsibility they will have to go to court and the persons mother and father both have to sign to allow them to be the guardian of the person.

Also at 17 in some states a person can just leave home as long as they are in and finish school, so it is possible nothing has to be done.

But unless they have the other parents permission, they can be in trouble.

And everyone had better know the reason also, since if the person needs other help, just taking them in is not always the best thing. If they just don't want to obey or listen to their parents and their parents are within their rights, this is a bad thing to happen