J.W.
First it is not fine just because they are over 18.
I have the safe house or I did until my older two got places of their own. If they won't call their parents don't get involved and def don't take them in! Even when there is something scary going on they should call their parents. It is not like an abusive parent is going to bust down your door and prove they are an abusive parent.
The kids that are having serious issues will call their parents. The kids that ran away because mom or dad wouldn't let them spend the night with their boyfriend Saturday won't call their parents because mom will be on your door step calmly telling you why junior ran away.
Just from what I read here you are not the safe house you are the door mat. Stop doing that.
This isn't a matter of legal rights, this is a matter of enabling children to manipulate their parents. Think about whether you would want your kids to do it to you? You set down the law so they go to another friends house to get their way?