This is a good question. Unfortunately, outside of the programming realm, there's no system for tracking changes at the character level that I'm aware of. (If there is one, I'd love to know about it!) I suspect there are many, many such systems that have been cobbled together by individuals, however.
This comes up often when blogging: Do we use Word files with their awesome change tracking features and then deal with all the garbage they produce before posting (possibly introducing errors)? Or do we work in plain text from the start, cobbling together some sort of change system like the one Michael suggests?
If you absolutely need something like this, I recommend using marks that are catch the eye. [[[triple brackets]]] or ***lots of asterisks*** or ###other such signs###. (If you're working on source code or HTML, this could cause problems later. And, as you can see, sometimes lots of asterisks render as bold-italic text, indicating yet another problem.)
Rather than embedding changes in the text itself, is it possible you could simply use versioning and the name of the person? For example, SampleFile_EditorNF_v1.0.1.txt may have meaning if you and your colleagues have agreed on a system. You could then use the compare revisions of a robust editor (such as BBEdit) to compare versions and see the changes.
Another option is to use commenting to describe the changes.