Simple Program Installed to Share A folder between 2 or more users

Hi Coders, This must be your lucky day! I have a need for a very simple project for someone that knows what they are doing. I simply require a desktop program that will share 1 folder between 2 or more users and keep the contents of the folder syncronised between all the users automatically. Connection between users must be secure an have some sort of login or authentication to join the sharing network. Users will be in different cities/countries and not on any network. All files should be transfered over the internet. No data is to be stored on a server online. All Files will be stored on each users computer. We also have a VPS hosting if needed. Going to choose someone fast on this, please bid right now.