Since I do not know your network setup it is hard to recommend any direct solutions but maybe, if it is not too late now, I can point you in the right direction.
If you have confirmed that users going home or getting offline corrects the problem you truly need to analyze your Network. Depending on your network equipment there are numerous tools already available for free to finds out where all the bandwidth is being used from. Here some examples:
1) Cisco routers have a nice simple little feature called IP accounting. You can enable IP accounting on your LAN interface and actually see where all the bytes are coming from/to. Once discovering who is talking more than expected you can go and investigate the individual machine or list of machines.
2) Lots of equipment comes with SNMP features. If you are running with SNMP enabled on your LAN and WAN equipment then you can download a large variety of useful SNMP tools that will watch statistics and pint you in the right direction of any unusual communications. There are numerous free SNMP tool such as MRTG (http://people.ee.ethz.ch/~oetiker/webtools/mrtg/index-2.html), Cricket (http://cricket.sourceforge.net/), and customizable tools such as UNIX/Linux Net-SNMP (http://net-snmp.sourceforge.net/).
3) If you have a hub or switch (Supporting some port mirror features) you can buy Networking Tools that will specifically be used to benchmark and monitor your network. These tools will often auto-discover every device on your Network and more or less monitor them based on how you setup the tools. One package that comes to mind is those offered by Solar Winds found at: http://www.solarwinds.net/Toolsets.htm. I have used their packages before and been happy with what I saw.
Hope it helps and let me know if you have any questions.
Jim
