G Suite and Office 365 Admins are not automatically identified as Admins in Dialpad. It takes only a few clicks to authenticate as a Dialpad Admin manually, though.
From Dialpad.com, navigate to Admin Settings > Users > Select Them From Your Directory.
Dialpad will redirect to G Suite or Office 365 (depending on which is used). Enter login credentials and allow access to Dialpad; you've then successfully authenticated as a Dialpad Admin.
Return to Dialpad, and you'll see the option to add users from your G Suite or Office 365 directory.
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