There are ways to see who is on the network either through IP Scanners, or like Michael mentioned by looking at the router to see what IP's have been handed out. If they have set their computer to connect with a manual IP you won't see that though, but may still be able to see who is connected wirelessly, and the IP scanner would show them too. Without running a server that is directing network traffic I don't know of any way to monitor actual throughput bandwidth though.
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