Add it as a target dependency. See this question for assistance with that: Xcode : Adding a project as a build dependency
Then you will want to change the Header Search Paths so that you can reference your interface files (the .h files). This setting is in your project settings under the build tab in the Search Paths section. You will set it to where your interface files are relative to your current project. So if the base project were in the same folder you would just reference them like:
SomeProject/Classes
If you need to reference a directory outside of your own you can do something like:
../../Projects/SomeProject/Classes
Each ../
represents a parent directory
Then to use your base classes, just reference the .h files like normal. Keep in mind, however, that your AppDelegate class is it's own class and you will need to recreate the app's initial display (using your base classes) in this file (just duplicate the base class' appdelegate code).
These instructions aren't how I would do it were I starting with this idea in mind, but are provided to help you where you are. If I were starting from scratch knowing that I was going to be reusing a lot of my code, I would create three projects. One for shared classes that I would reference using the above techniques in the two separate apps that use them (each their own project).
This isn't great, but it's still better than just copying the whole project and having to maintain them both for each minor change.