Improving developer experience in Open Source Projects

What makes an open-source project successful? How hard is it to add external people to an open project?
We at Nethesis have open source in our core with more than 400 public repositories that hold all of our company code. With the upcoming release of NethServer, we’re facing the excruciating question: how do we make it more appealing for developers?
Follow me in the changes we want to make to our processes to ease development and contributions, using big projects as examples and how even you can make a difference in the smallest of all codebases.