It was common in the Middle Ages for many people to work on a single page. Often one person, usually the guild master would decide the layout of the page. From there the jobs became specific. One person would paint the opening illuminated capital and another would write the text. A different person could apply the gold leaf. Then comes the painting of the borders, backgrounds for miniature scenes and people themselves, all done by different people. As you can see it used to involve many people after a page was commissioned. Now it is usually done by one or two.