You have to use QoS. Tag traffic coming from each user with a different traffic selector (this can only be done based on the source IP or if you use the Astaro Authentication Agent based on the then-resolved user name), assign the traffic to different bandwidth pools. Done. Make sure to create the bandwith pools on the LAN facing interface (if you want to limit download speed) respectively on the WAN interface (for upload limit).
If you have further questions, go ahead and ask more specifically.
