Your domain controller is of no use if you haven’t added machines to it. So here I am going to show simple steps to add client (window server 2008 r2) to window server 2012 domain.
From the AD users and computers of server 2012, right click computers, then go to New|Computer. In the prompt asking for name, give name of your client machine.
Then switch to client machine, and make sure in network settings, DNS is pointing to the correct DNS server. In my case, DNS address is same as IP address of server 2012. To make sure, ping your domain from client machine.
Now, in the system settings of window server 2008, click ‘Provide computer name and domain’ and then ‘change’ (Pic. 2).
In a prompt, select domain, and correctly put the name of your domain in space provided and press OK (Pic. 3).
Congratulations! You just connected your client to your domain.