Grant Privliages Based on Unix Groups

I am trying to setup readonly access to my user groups. I would like to grant select on a set of tables based on what Unix group they belong to. This way when new users are added to a ronly group on the ldap server, I don't have to go to each data base to update a local group on each postgres data base.

I realize you can create a groups and users in postgres and add users to groups. I would like for postgres to grant or deny access based on the users Group ID on the system vs having to create and manage groups on each of my postgres data base servers.

Current version running is:

psql (PostgreSQL) 8.4.4
contains support for command-line editing

I have been searching but can't find a reference to using unix groups to grant select, only references to groups created in postgres.

Thanks in advance.

John