There are many people using Composer already, and quite a few of them are contributing.
The most important guidelines are described as follows:
All code contributions - including those of people having commit access - must go through a pull request and approved by a core developer before being merged. This is to ensure proper review of all the code.
Fork the project, create a feature branch, and send us a pull request.
To ensure a consistent code base, you should make sure the code follows the PSR-2 Coding Standards.
IRC / mailing list
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