If you're the host and it's one of your customers, you're responsible for acting on a DMCA complaint issued to you about the article. When you receive the complaint you must act expeditiously to either remove the offending content or issue a legally binding rebuttal statement, declaring that you are within copyright law. Failure to do so makes you, the host, lose your safe harbor. Basically once you know about it, you're responsible for acting on it. If you don't, you're responsible for the content itself.
If you're knowingly using a plagiarized article yourself, then on behalf of whoever put the hard work into creating it, please stop.
And, with all advice on legal matters, what is important isn't want is said here - it's what your legal council tells you. We can give general advice, but we can't represent you. ;-)