I don't think they hide their existence from each other however. If they're like Unix users, then one might see something like /home/user1 /home/user2 /home/user3, etc. so that all usernames would be clearly visible and the user could be then forced to reveal all passwords. The aim is to obtain plausible deniability, that is logging in as the safest user according to the situation, while at the same time hiding all others.
