If it is your daughter with this problem, and presumably she has moved out of home (since she has roommates) why are you looking into this? Its always harder to do these things when the person investigating is one step removed from the problem.
Anyway, I don't know if BackTrack should necessarily be your go-to solution for this type of problem - its a Penetration Testing distribution, not a Network Monitoring distribution - and that means that the people here are more skilled in Penetration testing activities rather than Networking usage monitoring. As for suggestions, about all I can do at the moment is throw a bunch of names of open source network monitoring/statistics gathering tools at you for you to investigate - Nagios, sancp, ntop, Argus, Etherape.
Another suggestion is to get each user of this shared connection to install and use their own monitoring software on their own PC. If they all share the connection surely they all have a vested interest in finding out how much capacity each of them uses? Free tools are available to do this - just hunt around.