You can't learn C++ from diving straight into OpenNotrium, same as you cannot learn English by starting to read Shakespeare or trying to write a book.
(Please note that I don't think OpenNotrium can be compared to Shakespeare. Shakespeare doesn't have enough aliens)
However, if you have a grasp of basic things, you will learn a lot by experimenting with a real project, and I think OpenNotrium is pretty okay for that. The code isn't super clean, so don't think that what we do is the way to do it on all you projects from now on, but apart from that, it's probably in better condition than many other potential learning projects.
Git and Github made it really easy to collaborate and learn. Because we work pull-request based, you can't mess anything up, and we will be able to talk about the code.
Personal story time: I don't know much about C++, to be honest. I'm learning a lot from E_net4 and the reality of trying to get things working. I ask silly things about probably basic things, and he offers his point of view, and I learn. It's sometimes frustrating, but then it's really cool when it eventually all clicks together.
So if you know your way around Github and pull requests and issues etc, you can jump right in: https://github.com/verhoevenv/OpenNotrium
However, I guess it can be quite the barrier if you don't know github best practices. If you have any questions, I set up a public chat room about the project: https://gitter.im/verhoevenv/OpenNotrium. It's persistent and everything (which is why I'm not using IRC), so that means I might not answer immediately, but I will definitely see your message and try to help you. Or you can ask around on these boards, someone is bound to show up and help you around.