If you are a ROS user, we hope that you will consider participating in the ROS community. There are many ways that you can contribute, all of which help create a better software platform.
Ask Questions, Suggest Improvements
Even if you don't want to share code, you can help us make ROS better by asking us questions when you have problems. You can do this by asking a question on http://answers.ros.org or filing a bug ticket.
Bug tickets are preferred if you have a good description of the problem. Feel free to also suggest improvements to ROS and related libraries.
If you fix a problem in ROS or a ROS Package, please file a ticket or even better open a pull request. If you don't know exactly where to file the issue please ask a question on http://answers.ros.org.
Anyone can edit the ROS wiki -- you just have to register for an account (this helps to keep the spammers at bay). You can help us improve the Wiki by fixing errors, adding tutorials, creating pages for ROS Packages that you have written, adding screenshots, and more. You can sign up for an account here.
The ROS community is a community of communities, each focused on different robotics research problems. Feel free to look through the various Repositories to see if there is a community working on problems that you're interested in. You can also post on https://discourse.ros.org our forum. If you're looking for collaborators for a specific project the ROS Projects category is likely the best place.
Whenever you create a new package, feel free to send an e-mail to the ROS Projects category on ROS Discourse. This lets others who are interested in your code find it and it also lets others off code to integrate with it.
Of course, in order to share a package, you need a place to put it. As part of our open source principles, we believe in letting you stay in control of how your code is designed and developed, so our first recommendation is to setup your own Repository. Many other academic institutions and research labs have setup their own Repositories on SourceForge and Google Code.
If you're not feeling like setting up your own Repository but still want to share your library, please post on https://discourse.ros.org and we'll see if we can find a home for it.