Hi I had searched and banged my head for a while and finally figured out a way to authenticate users on squid3 using group authentication
for eg:- a group in ldap INTERNET having few members will get internet access rather then all of them.
auth_param basic program /usr/lib/squid/ldap_auth -v 3 -ZZ -b "dc=intra,dc=example,dc=com" -f uid=%s -h ip/hostname of ldap
external_acl_type ldapgroups ttl=5 %LOGIN /usr/lib/squid/squid_ldap_group -v 3 -ZZ -b "dc=intra,dc=example,dc=com" -f "(&(cn=%g)(memberUid=%u))" -h ip/hostname
acl ldapgroup external ldapgroups internet
auth_param basic children 200
auth_param basic realm test-Web Squid3 Proxy Server
auth_param basic credentialsttl 1 minute
acl ldap-auth proxy_auth REQUIRED
http_access deny !ldapgroup
http_access allow ldap-auth
http_access allow localnet
cache_peer localhost parent 3129 0 no-query proxy-only login=*:nopassword
http_access deny all