It’s for secular girls, but they often have BY or Yeshiva graduates. However, it’s possible that this has changed. You would have to ask them. I would think that a BY graduate could always go even if turns out that it is not so common, but again, you would have to ask.
Even if it turns out that it’s less common today (which I’m not sure about), one fact still remains the same and that is the fact that there is a tremendous amount that BY graduates can get out of it. That is why they used to go (and perhaps still do). They often feel that they learn things at Neve that they never learned in Bais Yaakov. I have heard this from one more than one BY grad who went to Neve.