This is the place to post questions and discussions on usage and style. The members of the Wordwizard Clubhouse will also often be able to help you to formulate that difficult letter.
When I read that statement, with no context, my first thought was "John knows where who works?... Mike?...Gary?" As Erik said, it's unlikely to be taken as in your second option and I eliminated your first because, really, doesn't everybody "know where they work"?ACCESS_POST_ACTIONS