A very simple way to control access to squid is to use the NCSA authentication module.
First ensure Squid is configured to use the NCSA module, for example put the following line in squid.conf
auth_param basic program /opt/squid/libexec/ncsa_auth /opt/squid/etc/squid_pwd
Then from the command prompt call htpasswd, to the password file, for the user
htpasswd /opt/squid/etc/squid_pwd user1
Re-type new password: Adding password for user user1
Then, you need to make sure that the password file is readable
# chmod o+r /etc/squid/passwd