Your users might have been fed up with proxy settings in browsers. If you want to get the internet shared via proxy without adding any proxy settings to your browser, just make your squid as transparent proxy following this tutorial.

Well first of all make sure your proxy is up and running. Now edit the http_port line .

[root@squid ~]# vim /etc/squid/squid.conf

http_port 192.168.85.1:3128 transparent

add ipaddress followed by port number with simcolun and a the word transparent.

Now it's time to nat, redirect all the http request to your proxy port, what ever mentioned in squid.conf file.

[root@squid ~]#  iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 192.168.85.1:3128
[root@squid ~]# iptables -t nat -A POSTROUTING -j MASQUERADE

That's it here your squid ip acts as gateway for your clients. Don't forget it.