Technically speaking, rule 3 specifies account sharing. if you want to give it away, and never use it again, that's not sharing, that's gifting. But if you ever want it back again, don't expect Inno to help you. And it would be the whole account, so any cities on any of the worlds, not just one city.
All you'd have to do is change the email attached to the account to theirs, then they could do a password reset and you'd never have to share your login information with them. They would never need to know your email address, and they would never need to know your password.
But if you have set up a forum account for that city, you have created a separate login entity and then you'd be getting messy.