i've been under the same circumstances before.

there are several ways to do these...
can i safely assume that you're on a windows environment in your network? why not just enable file-sharing option for a folder to make that directory and all files and subdirectories under it accessible to your network?
steps to enable file sharing within your network:
*just open windows explorer
*create a folder/directory whereever you want on your drive
*right click on that particular folder you want to share
*click properties, then go to SHARING tab, and proceed from there.

there's a WIZARD somewhere there to help you on this endeavor, and if you still scratch your head afterwards, don't hesitate to learn more sharing and security issues from the link somewhere on the bottom of the SHARING tab section.
that is configurable on your machine so if your machine is out of the network (turned off or unplugged), files will not be accessible to others. that's the downside.

but you can setup your central server (or ask your network administrator to set it up for you) to do the same steps as above (can be a partition this time and not just a single directory), the only difference being that the whole network can centrally add their files they need to share onto the central server, and everyone can get the chance to move them onto their own machines in a centralized manner.
after your directories/partitions are shared, just drop files you want to share on that part of your drive and they are accessible to anyone you permit to access them. this setup will not require you to use your bandwidth used to connect to the internet.

it's a local setup so it is a virtually instantaneous file transfer between drives in your network.
you can also setup a local network instant messaging system, with the file sharing feature.

that is if you're willing (and your network administrator permits) to test such system in your network servers. it can be prone to abuse though, since instant messaging can be addictive to some workers, though it can be very useful to a network with workers dispersed in their working environment. it can be an efficient tool when used responsibly.
i tried to google something of this nature, but i didn't test drive it so to speak. i found
Gossip Corporate Messenger (see why i am wary that it can be abused?
GOSSIP! 
) anyway, the page says it's a freeware and released 01-03-2005 (so it may be a little outdated at version 1;
paid version 1.2 is here.), but
if you can download the freeware, configure it for your network and it passes your network requirements, then good luck with it.

happy gossiping (and file sharing)!

if you're not up to purchasing a license (with the updated version), i'm pretty sure google will yield more freeware results of similar applications as the GCM. just search google for some alternatives.
NOTE: PLEASE BE AWARE OF SECURITY ISSUES PERTAINING TO FILE SHARING OVER A NETWORK. have a competent administrator to evaluate the security options available, and implement them properly. this cannot be stressed enough... so be forewarned of the risks involved in your network setup.
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