I have been thinking of writing this post for a while and here you go…
In vSphere 6.0 U2, you can have an External PSC or an Embedded PSC. The below process is to add an External PSC to the Active Directory Domain.
Login into the vCenter server, go to Administration tab, go to System Configuration –> Nodes and click on the PSC node you want to add to the domain.


once credentials are provided, click OK to proceed.
Note that the only way for you to know that this process is complete is that you get no error and there is no entry in the recent tasks tab in the vSphere web client. If that is the case then the domain add is successful.
Now, you will need to reboot the PSC

In a similar way, you can add the remaining PSC’s to the domain and finally, you will need to add the Identity source to the vCenter server itself under single sign-on