OK, so I finally was able to register yesterday and link my various accounts. As I am most active on Twitter, I immediately followed the first 2 properties on the Wiki list, the Anaheim Marriott and the Atlanta Marriott Marquis.

Today, I received 3 confirmation emails - 1 for @AnaheimMarriott, and 1 each for @RenHotels and @Marriott, which I have been following for months now. So, it seems you get credit even if you started following the accounts prior to the start of the program .