Are you looking to see where they are surfing, or are you wanting to measure bandwidth that is being used?
Also - what do you have as a connection to the internet? Too many unknowns to give a real good answer. There may be some freebies out there, and I personally know that there are some fairly spendy solutions, depending on your situation and requirements.
The only software I know of that runs on Mac and can show you what your employees are doing online is called My Team Monitor. You can register for free at their website (just google myteammonitor and you'll see the same words in the url of the website) and then your employees will need to install the software on their computers. When the employees start their shifts they turn the program on and it starts recording the number of keystrokes and mouse clicks they make and also screenshots of their screens every few minutes. This way you can see what they were viewing and what software they used, etc. This data is shown in an interface only you have access to.