Collaborative writing presents the problem of how to keep track of revisions. For UNIX users, there is CVS or RCS.
But a simpler method is to use some kind of scripting language like the bracket (to physicists: not "bra-ket") notation system mentioned here. The advantage is that you don't need any special software or text editor.
I myself have used something similar, though I usually use different symbols for deletions and additions.