Revision tracking and control
A necessary adjunct to the business of collaborative editing is keeping an accurate record of who did what to a document. In a non-real-time way, NLS as demonstrated by Engelbart had it.
Microsoft Office's Track Changes feature does too: As files get passed around, the comments, additions and deletions of each contributor appear in different colors, waiting for an editor to accept, reject or modify changes.
Collaborative cloud-based tools such as Google Docs, wikis and knowledge bases have embraced revision control even more tightly, with the ability to quickly roll back to previous versions of a file. It's in wikis and other joint projects that collaborative editing and strong revision control have found their strongest advocates. Where would Wikipedia be without revision control?