Good morning, I know it is difficult to let your children go into situations that you deem dangerous. The fact of the matter is that if you tell her no she will probably do it behind your back. Could you allow her to go for a small amount of time, increasing the time as she proves that she makes good choices. The other thing that I have learned as a parent of sons now 24,20 and 17 is that if they decide they want to do something they will find a way to do it, usually behind your back and increasing the risk of making poor choices. If you allow her to go on your terms, increasing the freedom if she makes good choices, decreasing the freedom if she makes poor choices would be a better option than flat out refusing to let her go.
My husbands father and step mother were very strict, his step sister was never allowed to go out in dressed in clothing that was to short, low cut, tight, etc... So she would put what she wanted to wear in her handbag, dress in the parent appropiate clothing to appease her parents then change when she got to a friends house. This is a prime example of finding a way to do what she wanted to regardless of what the parents said. I am also sure that she made some poor choices because she could not talk to her parents about the better choices.
It is hard to let your children grow up but you cannot keep them little forever. If you trust your daughter then show her by allowing her to go for a set amount of time, 1 hour, 2 hours etc... Explain to her that even though you accept her friends and their family dynamic, you want to keep her safe so you will allow her to go as long as the parent(s) are home, that she stays for a set amount of time then returns home and follows the rules that you set forth. By doing it this way you are keeping control by allowing her to go. You may find that if she spends some time there and is uncomfortable with the lax rules she may decide that she would rather not go. Good Luck