A free and open source extension for Pixelorama that allows multiple people to collaborate on a single project (by going to Edit > Teledrawing).
This is still experimental and it's not recommended to use this yet, other than just playing around.
Right now multiple users can work on the same cels, but in the future we should not allow this to avoid conflicts. Each user should only be able to work on their own respective cels, and operations such as resizing should probably be completely forbidden when a project has teledrawing enabled.