HelpNDoc is currently a single user authoring environment. We plan to investigate multi-user authoring for a future update but we can't say when that will be available.
Meanwhile, it is possible to link to external Word documents or HTML files which will be included at generation time by HelpNDoc. It is therefore possible for multiple people to manage their own documents and use one HelpNDoc project to include them when the final documentation is generated.
Here is a step by step guide explaining how this can be done: How to add an item to the library.