C# project - share files and other features

Collaboration software to sync workspaces (supporting network disconnect). I have 5 computers and I want my workspaces to stay synchronised across all 5 computers. I have one workspace that I share with 3 other people. Everything in this shared workspace will automatically sync with all the users (on all their computers). The software will copy customer files (dragged into collaboration software) in an encrypted store (such as VeraCrypt) I am working one one workspace with one vendor, I want the workspaces synchronised wth my computers and vendor's workspace. Workspace will allow features like file sharing, discussion, tasks, notepad and photos. The software will support multiple workspaces with chat and messaging. The solution must allow the user to continue working without network/internet connection and resync once connection established. The scope is left open on purpose to allow creative solutions and options. Only C# experts should reply.