I came across this great video by Mustafa K. Isik.
This was recorded a couple of years ago and I'm hoping the whole idea had developed since then.
Anyway, I searched for an Eclipse plugin that supports shared editing and I quickly learned about ECF:
However, despite well documented installation, I found no help there whatsoever to start actually using the plugin. Spending some more time trying to find this information resulted in this excellent how-to article:
I really recommend seeing the video in the first link, if you haven't done so yet. And then, actually giving it a try!