Hello Xavier,
I believe the reason no one is responding to the question, is that this has not been done before ;)
Opening up the workspace People to managers will give them a lot of access to employee records, that can be controlled quite difficult. Therefore I always advise my clients not to do this.
Annyway...
You need the privilege "View the people hub workspace" which is part of 2 duties:
- Inquire into workforce management proces
- Enable workforce management process
Those 2 duties contain a lot of HR processes.
If you would only give the privilege as mentioned above, this will result in a blanc screen:

You would need to add specific parts of information to be added (for example personal information).
But keep in mind, the acces for the manager will not be restricted to his hierarchy, but to the legal entities he has access to..
With regards,
Erik de Boer
Please mark as verified if the answer is helpful, it would be greatly appreciated.