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:
more generally, look at content management systems