Is there any way to monitor a wireless networks overall data transfer?

I’ve allowed our neighbors downstairs access to my wireless network. They insist they don’t use torrent or other P2P clients, but some of the internet speeds I’ve been experiencing say otherwise (I have Comcast broadband).

I know how to monitor the machines incoming/outgoing network speeds, but its there a way to monitor the entire network’s speeds?

I use Macs and have an Airport Extreme.