Support for 'list members of group(s) only'

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This seems like a useful feature. I think it can be implemented much simpler just using Vala code.

Could you try lp:~robert-ancell/unity-greeter/group-filter and see if that works?

The setting is enabled using GSettings which is the method for the other Unity Greeter settings. The easiest way to set these is to make a GSettings override file, see http://askubuntu.com/a/177024/28129 for an example.

And the contents of the override file should be something like:
[com.canonical.unity-greeter]
group-filter = ['group1', 'group2']

(unsubscribing sponsors - thanks Robert for handling this)

Robert, yes, it works and is what i want, exactly.
There is small issue, however. - Primary groups didn''t supported in current code. So, for example, if user1 has 'group1' as its primary group (set in /etc/passwd) he will be filtered out. Can you please improve in_group () method with smth like:

    private bool in_group (string group_name, string user_name)
    {
        unowned Posix.Group? group = Posix.getgrnam (group_name);
        if (group == null)
            return false;

        /* Check if user has group_name as it's primary group */
        unowned Posix.Passwd? pw = Posix.getpwnam( user_name );
        if( pw.pw_gid == group.gr_gid )
            return true;

        /* Check if group_name is user's auxilary group */
        foreach (var name in group.gr_mem)
            if (name == user_name)
                return true;

        return false;
    }

This bug was fixed in the package unity-greeter - 16.04.0-0ubuntu1

---------------
unity-greeter (16.04.0-0ubuntu1) xenial; urgency=medium

  * New upstream release:
    - Add an option to only show users in a list of specified groups
      (LP: #1493335)
    - Use non-deprecated Gdk.Cursor methods
    - Remove obsolete CONFIG_FILE define
  * debian/rules:
    - Update greeter logo for Xenial Xerus.

 -- Robert Ancell <email address hidden> Tue, 27 Oct 2015 11:48:39 +1300