Sharing folders in virtual box is simple and straight forward. There is a separate tab called as Shared Folders in Settings window, which allows you to share any folder from host machine to guest machine. Follow these steps to set this up.
1.Ensure that your VM is switched off. (Changes to settings are not allowed while the VM is in running mode)
2.Right click and select the Settings of the desired VM.
3.Navigate to Shared Folders, click “+” button to select any folder from host machine.
4.Select the folder in the popup and provide a name for the folder. We would be accessing this folder in guest machine by this name.
5.Fire up the guest OS and login into it
6.In guest machine add a new network drive and provide it's path as \\vboxsvr\[Folder name] . Folder Name is the name which you have provided while creating the share folder and vboxsvr is a constant used by Oracle Virtual Box to refer the host machine.
7.This adds a new network drive to your guest machine and will enable you to transfer file from host machine to guest machine and vice versa.
Note: These steps are based on my SharePoint 2013 development environment, with Windows 2012 Server as host operating system and Windows 2008 R2 as guest operating system for SharePoint 2010 projects