You get get such program by writing it. ;-)
I think you failed to find such thing because the while idea of multi-user collaboration at the level of the text editor is
very impractical, so people don't try to do it. As to my personal opinion, I think this approach is totally
wrong. Look at Microsoft Word. It is designed to provide multi-user access and collaboration, so what? who is using it?
I would advise you to look at some
very different way, such as
Wiki. Please see:
http://en.wikipedia.org/wiki/Wiki[
^],
http://en.wikipedia.org/wiki/Comparison_of_wiki_software[
^];
more generally, look at
content management systems:
http://en.wikipedia.org/wiki/Content_management_system[
^].
Good luck,
—SA