I had a similar request and made som thoughts about it, but haven't prototyped it. But i think you could use User-Groups in ActiveDirectory and if you give a User-Group Permission in NAV you can see this group in the dropdown for sending Notification to User. then in the OnInsert() Trigger of the Notificaiton Table you could check if the selected User is a Windows Group and send to all members of that group the notification by copying the same notification but with the Users that are members of that group (probably with .net interop get the AD Group Users, or just create a NAV Table for this with a list uf users per AD Group).
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I hope this can help you find a solution.