Comment 3 for bug 880104

I have exactly the same issue. LightDM does not add users to local groups based on the rules in /etc/security/group.conf. You only get groups from the LDAP directory. However if you login with ssh to the same workstation you get all the local groups from the group.conf file just fine.

I guess I'll just switch to KDM or GDM until this gets fixed. It works fine to use KDM for the login and still select Unity for the desktop. All my groups appear properly when I do that.