Phobos is still in it's early stages (i would say anyway) and i plan on refactoring several things multiple times over before I'd really suggest anyone to contribute to the project through PRs (pull requests). I have not written down the ideals and goals behind Phobos yet either, but one of the big points is to not feature creep. It's already a giant project in terms of ideas and plans.
The most polished parts right now are the tokenizer and parser, but even they are going to change with language extensions.
There's also no style guide, I'm not even 100% consistent myself, and I'm kind of waiting on having a formatter for it. I know there are some out there, but I'm not interested in using them. If I'm going to use one it'd most likely be one in Phobos itself, but that'll be a while since is not high priority.
So all in all I'm suggesting not to contribute at the moment - which will be a long moment - because doing so is very time consuming for you, and then there isn't even a guarantee that I'd merge it. I'm still learning how to work on projects together and it's proving to be difficult.