How to include multiple Xcode projects in a master project

I recently asked about the wisdom of using multiple Xcode projects to separate project components. I decided to follow this route in the next project.

I tried adding another project to the main Xcode project by dragging it to the sidebar. However, when I # import any of the classes that are defined in the added project, they tell me that these classes cannot be found.

What do I need to do to be able to refer to classes in other projects in the master project? I was surprised to find almost no information on how to do this on the Internet.