How do you trust people again? The answer is easy. Make them earn it. Realize not everyone is going to be your friend or looking out for your best interests.
Aside from the fact that this guy Sam assaulted you- he POISONED you. Yes- POISONED. Not only did he do it once, but twice- and stood by when his friends exploited you.
I can't tell you what is right for you to do. I can only tell you what I would have done. This subject came up when my ex told me he and his sister in law had talked about drugging me with GHB, so that we could have a three-way. They both knew I would never had done it other wise. I was LIVID beyond belief and you can bet your bottom dollar I would have gone to the hospital and contacted the police should that rape had come to pass.
In most states in the U.S. it is considered rape if someone has sex with an inebriated person. This is because the inebriated person is incapable of giving consent because of being judgement impaired. This actually happened to me. I was out with friends and drinking. I woke up in the middle of the night with a complete stranger in my bed on top of me. Sober, he was not someone I would have slept with. He took advantage of my inebriated state; in hindsight, he even contributed to it by buying me shots of whiskey.
I really believe this person doesn't see that he didn't do anything wrong and will most likely do it to some other girl. Do you want to be responsible if he does and finally ends up drugging someone to death?