I joined second NethServer to AD/DC NethServer by making the DNS server on the second NethServer pointing to the IP address of the Samba directory and not the IP address of main NethServer.
Main NethServer - 192.168.2.100
Samba AD/DC Nethserver - 192.168.2.101
Second NethServer - 192.168.2.102
DNS of the Second NethServer - 192.168.2.101
After I rebooted the users are now showing in the second NethServer.