How to configure zimbra ldap authentication on reverse squid proxy

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i want to create two group one group will have a access of webmail from outside of network and grop member will get a access after successfull ldap authentication on reverse squid proxy.
and i would like to restric other group to access webmail from outside of network.
kindly letme know how the above things can be achieved.
and also tell me the configuration of zimbra ldap authenticaion on reverse squid proxy.