From what I gathered (ie. talking to mraleph) v8 is mostly just Google employees. I can't speak about Chromium. The contributor situation is not vastly different for SpiderMonkey or JSC. I am usually the only contributor, besides 1-3 others that sometimes work on SpiderMonkey.

Edit: I think some ports are maintained from people outside Google. It's not really easy to tell, because most people have @chromium addresses.