Traffic Shaping

So here's the scenario, we run a public wifi network in our building for guests (mainly students) - they all use the same account (the only one currently set up). The overall bandwidth is then limited to 256kb.

However staff also use this network on their phones and laptops which we are of course fine with, but we'd like to be able to give them more bandwidth than we allow currently. Is there a way of creating two generic captive portal user accounts, one that is limited to each machine having only 256kb; and another that is not throttled at all (or at least has a more generous bandwidth allocation)

Any info at all on the subject would be much appreciated.