I’m not saying that the other posters are incorrect, but the way I read what you want to do is that you want to share the folder on the Host where the USB drive is mounted, not directly mount the USB drive on your Guest. To use the VBoxManager Shared Folders you need to be a member of the “vboxsf” group (the virtual box shared folders group).
If that is the case, the instructions on the website you visited were ‘almost’ correct. The command is:
sudo gpasswd -a $USER vboxsf
Then you can use the navigation functions of the VBoxManager Shared Folders to go to your mount point on the host and mount the contents of your USB that you mounted on the host.