If you mean to contribute actual code, the place to start is: http://labs.jboss.com/contributor/
Once you have done that, we can use the patches you submit through JIRA. Regular contributors can also get commit access, but that usually only happens when you contribute so much that we get tired of spending all our time committing your patches instead of writing our own code. :)
We definitely welcome anyone who is interested in Seam to step forward and contribute. We're a very open project.