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.
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.