If you start Squid as root, it changes the process’ user ID to the username specified by cache_effective_user. By default, Squid sets the process’ group ID to the group associated with the cache_effective_user. You can set the cache_effective_group directive if you want Squid to use some other group ID.
You only need to set this directive if you’re starting Squid as root. If you start Squid as a non-root user, this directive is ignored.