No Notice to Site owner when Sharepoint administrator add new user in Sharepoint Online

Vivi 

This is super late, but there is a solution to this.

You'd have to make this HR person an admin, but you can make it so they're not a global admin, so kind of weaker admin so they won't have too much control. 

Then, Admins can also use alert policies to monitor activities like assigning admin privileges. Alert policies can be created in the Microsoft Purview compliance portal or the Microsoft Defender portal. 

You can create an alert policy for this specific action and have it alert their admin type.