do you mean authentication or access control lists (acl). Unless you've specifically set squid up to authenticate clients based on a username/password, squid won't authenticate out of the box.
If you're getting an acl error, take a look at squid.conf and search for this line (comes in the default config)
acl localhost src 127.0.0.1/255.255.255.255
beneath this, add a rule for your LAN, for example :
acl local_network src 192.168.1.0/255.255.255.0
then, *before* this line :
http_access deny all
http_access allow local_network
and reload squid. Hope that makes some sort of sense, or fixes your immediate problem.