With NetWorx you basically have got two options:
1. Install NetWorx on every computer on your network.
This can only be done if all your computers are running Windows.
While NetWorx can only monitor traffic on the computer where it is installed, instances of NetWorx can talk to each other so that each instance is synchronised and shows the actual overall usage. The synchronisation needs to be turned on in the
advanced options.
2. If your router supports so-called SNMP protocol, you can monitor all traffic via your router. This however will not give you a per-user usage breakdown and the monitoring computer must be continuously on to poll the router and collect usage data. Check your router's documentation with regards to SNMP support.
Hope this helps.