Bi Directional folders (Mac to Mac) Currently we are running a system where a group of 12ish Apple Mac computers connect to a Mac Mini Server. 

We also have a few off-site teams who need to access said server as well. We would like to know if there's a modern system that allows the user to drop files into a folder which then shares with the server and also shares with the off-site teams. (obviously allowing time for upload/download time)

This would need to be peer-to-peer we do not want to use Cloud Computing.

An example of this being used would be:
Folder on User 1's desktop is populated with files, the same folder appears on the server which also appears on user 2's desktop. Only when the files are requested should they copy from User 1's machine (it would be a 24/7 machine).

I'm aware there are other questions like this on the website already but I couldn't find a definitive answer. We are looking for a professional solution and not necessarily a work around.

Thank you,

Ben
Yes.  It's called cloud storage.

The best one I have used to date is OneDrive for Business.  This does exactly what you want - which is to drop a file in a designated folder and then it will propagate to all with access to that folder.