Your contribution is valuable for Radiance and welcomed by the maintainers. The best way to get your ideas into Radiance depends on its type:

bugfixes check the mailing list archives or use the Radiance search to check whether your problem is known already (it might be a user error after all).
email to the Radiance mailing list
That requires subsribing to the list, which you should do anyway, as a serious Radiance user.
If you really want to be top-notch, prepare a little demo scene or image that shows your problem clearly, put it on the web somewhere and include the link in your posting to the list.
documentation and examples write html pages, pack a zip or tar file with the pages (and images) and sent it to me to put it on the server. This account doesn't accept HTML formatted emails, so sent your email with attachements, in 'plain' mode. Your pages will be valued by your fellow users, and you may sent updates.
scripts and stand-alone programs (converters, generators, etc ) Try to compile C-code on more than one platform (three different opsys help wonders, compared to only one opsys and compiler). Write a man page, wrap all that is needed in a tar file, contact Greg Ward on the mailing list and chances are good that it will be included in the distribution and valued by many.
changes or features to the core, e.g. rendering engine and libs Be prepared that your changes will not be added to the code ad-hoc.
See various discussions on the dev list why that is: Feb 2005 email thread Hi / feedback of code and Dez 2003 Quo vadis Radiance.
If you're really into it, start from the latest HEAD release, add what you like and offer your changes as patches to the community. Sent a patch file to me to put it on the server or put it on the web somewhere else.
Try to compile C-code on more than one platform (three different opsys help wonders, compared to only one compiler).Maintain it. After a year you are likely in.
direct CVS There's no direct CVS-like anynoumous read or write access, read access is offered via the web interface, write is done by the core team of maintainers.

Peter Apian-Bennewitz,
updated: