We finally found it!
It's here, in case anyone was wondering: https://itch.io/dashboard -> More -> Contributors
You add the name of your co-creator and press the button "Add admin".
This will send an invite that the co-creator has to accept.
Then, the other person can manage the game's page, edit it, upload versions, add descriptions, see analytics, etc.
As if they had created it themselves. and were seeing the page from their account page.