Use of the "Everyone" group is not recommended for security purposes.
The correct group to be using is the 'Users' group which includes a build-in called 'Authenticated Users'. All processes should run as an authenticated user on the system, and therefore be granted access.
If you find it necessary to add the 'Everyone' group to a folder to get MP2 to work then there could be something else is wrong with your machine.
Please re-test with 'Users' instead of 'Everyone'