My son, at 16, is slender...it is the build he inherited from family...so he will never be a bulky man.
He is, technically, underweight, always has been.
When he was about that age his pediatrician wanted him to pick up a few pounds so he would at least be on the weight/height chart. Since we have always maintained a fairly healthy diet - no soda, limited junk and processed foods, and he was a picky eater, getting him to increase his food intake just was not going to happen. I added Carnation Instant Breakfast to hi milk - half a packet with breakfast and the other half with dinner. He picked up a few pounds, his doctor was happy, and all was well with the world.
If your daughter is healthy, but slender, I really would not worry about it. My son is perfectly healthy and strong, just slim. Oh, and he takes Focalin, Zoloft, and Prevacid daily , along with as needed over the counter meds.