How should B verify their email? I guess by clicking on a link they receive via email? In my test, B received no mail. I double checked in spam folder; I've tried with A and B two gmail addresses.

Something else: Let's say I am 84 000th on the list, and a friend joins in, so I am 83 999th. Then during 3 days I don't do anything. Question: Will I stay 83 999th or are new people arriving in the meantime and sharing a lot going to pass me, and I will finally be moved back to 90 000th?