Several local clients shall sync locally

Our OwnCloud Server is hosted on the webspace of an internet service provider.
It works very well with several users, syncronizing files on their laptops as expected whenever they get a connection to www.
But not all of these users are always travelling, the one or the other will be at the head office. Our network provides internet access. So the following happens: Person A edits some files that are correctly synced to the OC-server. The OC-clients of Persons B, C, D etc., they are all users of the same network (!), each of them syncs the files separately. So the files are sent serveral times through our internet access point, putting a heavy load on it.
How can I manage that the OC-clients sync locally before they use a WAN-connection?
What kind of setup is needed, that they ask their neighbours before looking at remote places?

Thank you!

The feature request is called "LAN sync" and is currently a wish but not implemented nor worked on anytime soon. You are welcome to contribute in whatever capacity you like, including Bountysource: