Yes but if Mickey Mouse or Donald Duck were served with a subpoena in California, they would become involved in the lawsuit to which they are not parties, to the extent that they are obliged to comply with the subpoena. The question is whether the business records subpoena in California is valid...
Let's say a lawsuit is filed and one of the defendants is never served with the summons and complaint and never dismissed, but documents are needed from this unserved defendant. Clearly, a Demand for Production of Documents under CCP 2031.010 is not going to work, so a subpoena for business...