Depends. If you're running an http proxy, make it transparent and review the logs. Obviously that will only get you info about their http traffic.
If you want to capture everything, consider tcpdump or wireshark. Obviously those won't work on a switched network, unless you can configure your switch(es) to mirror all traffic to the sniffing box.
If you have access to the main router/gateway, it's probably easiest. Just configure it to log all traffic, either via firewall rules or a dedicated sniffer program. If you do that though, expect your performance to degrade slightly, depending on how much traffic you get.