If I knew somebody else's unique subdomain, I could set my browser cookie on my local computer to that value and it seems to just load the other subdomain just fine. I tested this with 2 different browser on my same laptop. Maybe it won't work if the other person is on another computer?

I could also just set the subdomain to anything I like (by setting the cookie value) and it still works just fine.

Ah no, I can still set the cookie to the other person's subdomain on another machine.

Edit: add extra sentence.

Edit2: format.