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Dear Sally - You are not responsible for the 18 year old but you can be responsible for the 17 year old. You do not have to allow the friends into the house however it would appear things are out of control. In order to protect yourself you can contact the police and explain the problem that you do not condone the drinking but cannot control a bunch of boys at your home. If they get charged with a crime then that's not your problem. You can file trespassing charges against the friends if you tell them to leave and they refuse to do so and you can evict your 18 year old from the house since he is an adult and has no rights to be there without permission.
Those are your rights but whether you want to go to those extremes is up to you. I am assuming the parents of the friends are not responsible enough to control their children but you can also contact them and report what's going on.