It has long been possible to give editing rights to users of an instance of the publishing application on a project-by-project basis, and each writing has an Authors field where one can tell who the authors are. Sharing session code after logging in is also a way to give access rights, but then one would be giving access to everything one can do with that user account. The internal functionality of the publishing application currently includes the possibility of realtime device-to-device communication between users of an instance of the publishing application, which, if further developed, could make collaboration between users more sophisticated than just giving editing rights.
These are somewhat experimental without well defined purpose, so everything related to multiusers (authors and co-editors) are only available via enabling the experimental features in the user settings.
A user can add authors to both their own writings and writings belonging to projects to which he has received editing rights. They will be used in contexts in which the authors' details etc. will be displayed. E.g. in the fine-tuning view, you can add the contributors placeholder to the writing, which will be used to generate author information in the final version of the writing. The function for this can be found in the Tools menu ("Functional embed: contributors"). That placeholder can be converted to plain text by Alt-clicking.
These "authors" can be created in the Authors tab of the user preferences, and they can be transferred from one user to another if necessary. They can be used to communicate to readers the role played by each author in the creation of the writing. Later on, images can also be used, but at this stage these authors are text only. Users with editing rights will not be able to remove authors other than their own.
These authors are loosely decoupled from writings in such a way that if a project is first exported and then deleted from among other projects, the relationship between them is broken. The backup will only include a hint of the author's name, but on the other hand there will be a semi-automatic function in the Usabilities view to reconnect authors.