Yes, Jupyterlab has a collaboration extension.
I've spent the past month writing a HTTP+WebSocket proxy to be used with Jupyterlab for an ESA-adjacent company, and spent a good deal of time trying to make the collaboration system work behind this proxy.
That said, I don't think my contribution is at all related to this post.
I guess things might get nasty when you try to _run_ simultaneously, unless they actually have implemented a CRDT for the whole notebook state (it would still be nasty from a usability POV, but at least it would be consistent). Just writing on the notebook would work just as any other collaborative editor out there, I expect.
JupyterLab does use CRDT via yjs (https://github.com/yjs/yjs), and the JupyterGIS collaboration system is based on the service provided by JupyterLab. So multiple users can edit the file concurrently.
What does this mean? How is it collaborative in real-time? (I don't even know how Jupyter is collaborative... as in, several people can open a Jupyter Notebook and make changes simultaneously, and things don't break for either of them?)