Caches pay off from scale, and 5 people each reading their own favourite sites is unlikely to really make much difference, especially with more and more sites being dynamic and preventing caching. It totally depends how your few users work but you might prefer to explore things like browser extensions, like noscript for firefox and other image downloading extensions to stop bandwidth being sucked up.
It's probably more likely to be worth your while using the analysis functions of squid rather than expecting an immediate benefit from caching (although if you do send urls to each other there would be *some* benefit.). y using a tool like squidalyzer you can see what sites / staff are costing you the most and just address it at a personal level. Do you allow youtube? If so cobblers to not downloading files... if they click the high quality videos that'll hammer you. and again IT ignorant / naive users might be watching live tv etc.