Sharing files between VirtualBox host OS and and guest OS

How to share files between host OS and guest OS in VirtualBox.
Host OS: Windows 7
Guest OS: Linux version Ubuntu

Create Shared Folder. Settings -> Shared Folders. Click on add icon, specify Folder Path and Folder Name, check Auto-mount. Folder Path is path on Host OS to be shared. Folder Name is a reference used by guest OS.
VirtualBox will create folder sf-[Folder Name] under /media .
There is a pitfall here. This directory is not accessible. Access permission to this directory is  not given by default to the user who install VirtualBox and set up Shared Folder.
Here is instruction of how to grant the access permission to this directory (see )

1. Open a terminal.
2. Enter "su -" (without quotes), hit Enter.
3. Enter the root password, hit Enter.
4. Type in "usermod -a -G vboxsf username", without quotes, replacing username with the user you want to add to the group, hit Enter.
5. You may have to reboot to remount the share, the easiest way to do this is probably to reboot the VM.