SpiderMonkey contributors generally hang out in the very active [irc://irc.mozilla.org/jsapi #jsapi IRC channel].
The [https://www.mozilla.org/about/forums/#dev-tech-js-engine-internals js-internals mailing list] is used for communicating with other SpiderMonkey hackers. (<i>Internals</i> as in discussing the internals of the SpiderMonkey engine. All are welcome on the list.).
Reading Planet Mozilla is the best way to keep up with the Mozilla project, which includes some SpiderMonkey related blogs: