There are a lot of people using composer already, quite a few are also already contributing.
If you would like to contribute to composer, please read the README.
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 Coding Standards which we borrowed from Symfony.
IRC / mailing list
The developer mailing list is on google groups IRC channels are available for discussion as well, on irc.freenode.org #composer for users and #composer-dev for development.