You don't.....
However,
When you join someone else's pbem game, the host will have set an admin password, and you are therefore relying on the host that they can replace any player that suddenly disappears by opening up his save with the admin password and then changing the player password ready for the new person that takes over the tribe.
We now know that Kenwyn forgot the admin password, and his statement on December 23rd was just a wild guess. Despite my question a day or so later, neither he nor Sinimusta actually checked at the time that this password was correct.
Sinimusta has posted in the replacement section, so I am assuming that SITS has provided him with SITS password, ready for the next player. However what we now know is that this game has an admin password without our knowing what it is, so we are at risk of dropouts and the game ending abruptly.
Having Sinimusta collect everyone's passwords and forward them on to Ainwood is some level of security against a future default. I see little difference between the previous idea that Sinimusta had an admin password, or a future idea that he knows every individual's passwords - either option gives him complete access to the game.
However, I have experienced enough seemingly reliable people disappearing that I don't want to risk another year or two playing a game only to see it stop dead. I personally need some guarantee that a game can continue, and normally I am implicitly trusting the host's reputation to provide that.
I don't want to coerce anyone into doing something they are uncomfortable with, and have enough other games going at the moment. If anyone decides they do not want to provide their passwords, then I am happy to advertise for a replacement and continue my turns until someone can take over my tribes.