That is a VERY good question, because that is actually a setting that is VERY WELL hidden in my opinion. I have tried to locate it in Power Apps, but not been able to find it there.
You can’t actually grant access for specific users, but you can set which user Security Roles that should be able to access it.
Here’s how you do it, assuming you are System administrator and have access to all the apps:
Open any app and click the header link to open the list of selectable apps:
Locate the app you want to edit in the list of published apps and click the three dots in the upper right corner (the “More Options” button):
Select “Manage Roles” from the menu:
In the Pop-up dialog on the right hand side, you can select which Security Roles should be able to access the app. You can also see how many roles all together that can currently access it.
Don’t forget to hit save when you have made your changes.