If your router can show the ARP table, then you could be able to see who accessed the Internet the last hour/day, depending on how long its caching the entries.
If the router supports SNMP, then you should be able to get the total ADSL bandwidth usage, but you would still be unable to track the individual usage, unless each person is connecting from a different physical source (eg. Ethernet Port 1 - 4 / Wireless).
I know that my friend's D-Link 2640U had SNMP, which allowed him to track the usage with apps like Traffic Grapher / MRTG.
An alternative would be to get a MikroTik router (like the RB750 for around R325 ex VAT), which you hook onto the ADSL router in bridge mode.
It has a built in User Manager that would allow you to monitor traffic usage, rate limiting, total download/upload limits. The User Manager could be used in combination with the DHCP server to hand out IP addresses to everyone and track the Usage on MAC Address level AFAIK. * Note: I've only completed the Beginners training, and I've only used the User Manager with the PPTP Server and not really with the DHCP server.
If the User Manager doesn't seem to do the trick, you could try the Hotspot feature instead, where the users authenticate their MAC address by logging in using a web interface. That way you don't need to add fixed MAC addresses. Instead you give everyone a username and password.
You should be able to allow the users to dial their own PPPoE connections from their PC's OR you could run multiple PPPoE connections from the MikroTik routerboard too.
I'm using a RB750G as a PPTP VPN server, where me and a couple of my friends share a 4Mbps ADSL line with both uncapped & unshaped accounts. We're using the User Manager to manage the limits and username/passwords of the PPTP accounts.
