There are three main ways to do that:
1) If your router supports it, you can get statistics from that. Many don't - contact your router manufacturer.
2) You can add software to each system to monitor it's usage:
Networx[
^] is free and seems to work - but I've not tried it myself and it only runs on PCs.
3) I believe you can also add a dedicated machine as a proxy which handles all the traffic, and monitor it that way.
You might be best looking at internet cafe software, and working out how that does it - as IIRC some did limit usage / bandwidth per user.
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