I’ve been watching this repo like a hawk since I’ve always wanted to start with this.
Do you know if this repo is open to anyone to join? I’d like to join and test stuff or help with it. Of course, I expect it might go over my head or not ready, but I’d like to try it out every new push. Do you know if this is open to public? 
As with all Gazebo projects, you are free to use them as you like. If you would like to help with the development of the project, you can fork it on github, make the changes in your fork and then open a pull request against the original repo, describing the changes you made and how to review them. The authors of the repo will then have a look at the pull request and can merge it if they consider it useful.
The repo is marked Public, so, as upstairs said, it’s open to the public 
Looks like it’s new and under very active development. The code will probably change very rapidly, i.e. expect new features and instability, as with everything on the bleeding edge. Since it’s so new, if you are interested in adding a new feature, it’d be safer to check if they’re not working on it already. It would probably be easier to test things out and make some fixes, until you have a sense of what the roadmap is.