Unfortunately, that error means exactly what it says. Those email addresses have been added to another organisation, which has triggered a verification email which the user has never clicked through to verify their email address. Until the email address is verified you won’t be able to add them to your organisation.
I’m not sure of the exact process to follow here, but I’d suggest you ask the users of the email addresses that are erroring to try and log into the Dashboard, see if they can verify their address that way. You can also try support, but you may or may not get any luck there (personal experience is they’ve been very reluctant to remove administrators, or change user details - probably for good reason).