In order to protect our repositories, only the offcial development team is able to push changes to our repositories. You can however submit patches in our forums and ask to join our team, we are always glad to welcome new developers.
If you find a bug, report it in our bug tracker. The bug tracker is also a good starting point if you want to help LuxRender development effort and don't know where to start looking at.
If you plan to develop an exporter for a new 3D package or extend an existing exporter, check the scene file format documentation or ask in the forum. If you want we can set up a repository for your plugin provided it is licensed under an free software license like the GPL.