how to restrict sites using squid

Dear all

I installed redhat 7.1 in my office.. and configured squid for sharing net access. is there is any option to block sites,,, ya, i know there is option, wat i want is only i want to restrict a group of people to access sites like yahoo,hotmail etc the same way how v r blocking machines from gaining net access….

pls. give solution….

regards
Sakthi