1. Click on VirtualBox VM Devices and Shared Folders... to create a shared folder, named it SharedFolder, for example.
2. On Ubuntu guest machine, open Terminal
3. Type sudo mkdir /mount/vboxshared
4. Type sudo mount -t vboxsf SharedFolder /media/vboxshared to mount the VirtualBox shared folder as read. To make it read/write, use this command: sudo mount -t vboxsf -o uid=1000,gid=1000 SharedFolder /media/vboxshared
If the shared folder is a permanent share and auto mount, make sure your user id in group vboxsf. To add a user id to group vboxsf, using command: sudo usermod -a -G vboxsf linuxuserid
Reference:
https://help.ubuntu.com/community/VirtualBox/SharedFolders
https://forums.virtualbox.org/viewtopic.php?t=3201&highlight=share+folder
