How to improve groups understanding of our complex codebase

Question

We've inherited a very complex codebase and only 2 out of 6 members understand the flow in detail. How can we improve the groups knowledge, where do we start?
We're talking 10s of thousands of lines of code broken into 2 main projects.
The code itself is difficult to understand, contains alot of Delegates which makes the Debugging a nightmare.
Creating new Design Documents has been mentioned but in my experience this is rarely of benefit as the code detail is much more complex than the design. I feel some description in english and Use Cases are what we need, but where do we start?