How to add multiple users to SharePoint alerts?

_MaherS Do you have full control permissions on these files? As a site owner, I am able to add alerts on file and other users to same alert: 

Try with higher permissions once and see if it works for you. If not, you can always ask other users to create the alerts from their end.

You can also check if new Rules functionality in SharePoint works for your requirements: Rules in SharePoint Online/Microsoft Lists 

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