My 28yr old son is living in my house. He is back on drugs and I am told by the police that I have to give him 30 days notice to get him out

You have to give him 15 days notice, unless he actually has a lease with you. Post it on your front door with the words "Notice to Quit" across the top. On the 16th day, go to your local magistrate and file for an eviction. Regarding his friends, no, they may not enter your home without permission. Threaten to call the police if they do not leave and then do it if they refuse to leave.