@J-3804 ,
You could only add the contacts one-by-one manually. It is a big task if your organization have so many users.
As a workaround, admin can create an org-wide team which will automatically contain all users in you organization. You could refer to the member list of that team when you want to contact some people.
For more information about how to org-wide team in Microsoft Teams, please refer to this article.
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