Arduino /dev/ttyUSB0 permission denied even when user added to group "dialout' on Ubuntu 18.04
After some struggle I figured out that I need to add my user to "tty" group as well.

There's another approach that involves udev rules modification for certain vendor:device and update of device's ownership but this seems more sane approach.

To recap: 


Add your standard user to the group "dialout'

sudo usermod -a -G dialout your-username
Add your standard user to the group "tty"

sudo usermod -a -G tty your-username
Logout/Login


Hope this helps