They have offered this for years. You need to meet certain spending criteria, but you should be able to call them and tell them you would like to create an umbrella login account for your smaller accounts (maybe it is called a master account?).
People say it is around 5k a month in total spend before you get approved, but I have never heard that number confirmed.
Spoken to them about it and they apparently don't offer this facility anymore. The only thing they could offer me was a sister account service. Which doesn't allow for the same top level management that google offers.