I use Google Docs for student sign ups, and I include this note:
All edits to this document are saved, so don’t delete anyone's name, because you will leave evidence! If you make a mistake, go to “Edit” at the top of the screen, then click “Undo.”
If you are worried about students being in the document at the same time, don't worry about it, because a colored line appears wherever somebody is actively typing in the document. If they're typing on the same line at the same time, they'll be able to see it. I've never had students actually manage that miracle of timing, though.