It really depends, dearest.
Underage is a whole different ball game from mature.
Underage sex isn't bad, per se, if the person having sex is mature.
And being safe.
And not sleeping with everything with something dangling between its legs.
If she's mature, safe, and in a good, loving relationship with a man (yes, man, not boy) that respects her and won't just go running off to his buddies saying to them "I scored with that chick, she's a nice piece of a**," then it's really not a problem.
However, since you're saying she's talking other friends into sexing people up too, I figure you mean she isn't ONLY underage, but also immature, and they're probably sleeping around and not using protection.
Maybe you should just express that you're concerned and want her to be safe about her decisions on sex?
Don't just tell her you think she's young and stupid even though I'm sure that's what you're thinking at this point, that likely won't go over well, but if you're really nice and just act like you're being simply a friend concerned for her safety, it shouldn't be too bad.
Hope that helps you a bit, hun.