Well, I too could not find it at first but then realised it’s might be something kinda subtle.
With all that hush hush… ;-), I’m guessin'
If you received that first mail where Ev offered you founding member status, and IF you clicked the link, THEN you would see a form to add your credit/debit card number to pledge your money. But also, you would see a few lines to pitch your ideas and three samples of your existing works/articles. It seems Aura Wilming refers to that extra offer that Medium would consider your direct writing for this new member only paid thing as “partnership”, I am guessing here.