For line (row) level locking, add handling to only allow one user to edit a particular line at a time.
For each user currently on the page, assign them a discreet palette color. Highlight the current line each user is on with their color to give indication to all other users what line everyone else is on/modifying.
Have a sidebar (collapsible/expandable) on the page that shows all given users on the current page, along with their personal color. This lets you associate live edits to a particular user. This would function much the same as using Google Docs (i.e. spreadsheets).
I shouldn't be able to hold backspace on the same line my collaborators are editing IMO. Another added bonus would be to do a very faint row level highlight to show where other contributors are. Lastly, just to show a list of the current page contributors.
