I have a regular, Un changed Samsung Tab A running the unchanged version of marshmallow OS (6.0). I have enabled installation from unknown sources in the "owner" user of the device, but still can't enable it in the second user (guest). I would like to install a .apk file which I have already tested in the owner user. In settings the "allow installation from unknown sources" is greyed out so I can't change it. Is there any way that I can install the APP?
Enable USB debugging as the first user(owner) and run,
adb install -r filename.apk
This will install the app for all users.
Hope it helps