Promiscuous teenager?

After finding out that my 15 year old daughter is promiscuous I am ata quandry about exactly what to do. Because she is my step daughter I have limited authority. My husband just gets really mad and hurt when this subject is brought up and her mother just seems to yell alot and then take her shopping. I wan to know what I should do that would be truly helpful. When I've brought up the subject with her before she just denies everything but I know it's true. I read her myspace messages. She is very bold and tells guys she will do favors if they come to visit. I know this is a self esteem issue and I consider myself pretty smart and understanding but when it's you own kid....it's really hard to fugure out the right path. If I had control over the situation, she would just not be allowed to go anywhere or be anywhere without supervision. But I don't have that authority. I also don't want to just tell her no and not get a better understanding of why this is happening. Any real insight?