@Derek1, if you log in by email and not via social media, you can log in simultaneaously from 2 computers or a computer and the app. I do that all the time, when I change my listings or post on the phone and continue on the laptop to write a longer text. If you trust this manager, you can just give him the log-in details, email used and code. Just do not choose a code you use for anything else. And most of all use different codes on airbnb to the codes used to read your mail.
You can set the airbnb account to send you notifications for access from another computer / phone. You can also check the history of access on the account. So if your manager works on a windows computer in his office in one town, an ipad at home in the region, you would still be alerted and could block that, if you get a notification that someone logged into your account at the other side of the globe.
