I agree with the others here as well. I would think that adding
co-workers on Facebook would be alright, especially if you are starting to become friends with some of the people you work with. But for me personally, I wouldn't feel comfortable adding someone who held a higher position than me on Facebook, not because I'm worried about what they would see, but simply because it could be seen as unprofessional. That is just something I really wouldn't want to get into. Like the others suggested though, if it is in volunteer position anyways, I don't see any problem with you adding her. You might want to ask her in person if she would be alright with it, but there is a good chance she would be fine. If she is a higher up than you and you do decide to add her, I would suggest hiding the things she could see so you don't come off too unprofessional, depending on if she was in management or not. Hope this helps!
~ Angelica