ContentSubmitters AD group: root domain or child domain???

Hi

We have an empty root domain.  Mailbox users & Exchange 2013 servers are in a child domain.

As per Microsoft's documentation; we want to create the "ContentSubmitters" group in AD for content index to work properly (article 2807668).  However I do not know where to create it!!!  The article doesn't address it.

Does it go on the root domain where default exchange groups reside OR OR OR OR OR does it go on child domain where exchange servers reside?????

Thanks