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Prior to designating an alternative host (see Fig. 1), make sure that the user has a Zoom account in the same domain. For example, if your account is in umsystem.zoom.us and the other person's account is in umsystemprotected.zoom.us you will not be able to add them as an alternative host. Please see this article from zoom for more information.
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Figure 1: adding an alternative host in Zoom meeting settings
Additionally, If a user has not yet created a Zoom account, you will get an error message (See Fig. 2). They will need to login to Zoom to create their account prior to your attempt at designating them as an alternate host.
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Figure 2: error message when the user account does not exist.
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