My first recommendation would be to encourage an existing member of the group who will receive notifications anyway to setup a forwarding rule in their email client so whenever they receive the notification, it gets forwarded to the other email address at the same time.
Alternatively, you could create a service account using the email address that needs to receive notifications, then add the service account to the workflow. This approach can become messy if you don't have good governance around the creation and usage of service accounts.
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