Each of your users will only use his own computer? Do you have to use user account credentials as a unique identifier for your plan or can you use the source IP addresses?
In the first case you can use the TC utility that comes with
Iproute2
, like described
here
and
here
. You might find convenient to use
netfilter
(through iptables) to match and mark the packets depending on their originating address.
In the second case, you might find the
NuFW
project helpful (you will still need familiarity with iptables/iproute2 however).
PS. Here's also two projects that attempt to provide a GUI for iproute's Traffic Control -though I'm not sure whether they would make things easier or more complex-:
-
MasterShaper
-
Easyshape
