I am glad to hear that you like our editor! As for your question about collaborate authoring: editing a single document with multiple users is unfortunately not possible at this time.
However, it is possible to simultaneously work on separate files that are in the same ditamap, which might ease the pain a little. The non-free licences of Fonto also include a locking mechanism, which notifies you when other people are editing a file to prevent overlapping changes.
I hope that answers your question.