And to answer my own question, there was no point in registering. All you have to do is show your Amex; there is no list.

No line when Mrs. Trojan and I arrived. About 10 min to order then another 10 min to get food. Bit crowded, but you could sit outside and they were also directing folks to the public park on the roof of the building across the street.

Pilot, two FAs and a LGA red coat circulating (Pilot and FA worked the door.) Very friendly people, represent Delta well. (Would like to have FAs like that on my TATL flights...have had mediocre crews in J all year.) Inside there were people from Amex asking each person if they were primary on their card, did they have a business or personal card, etc. Emphatic but friendly. Clearly this was the point of the event.