1. Create a folder in your Mac home directory, let's say ubuntu-sf.
2. Install guest additions. Click Devices -> Guest Additions and run the installer.
3. Reboot VM
4. Select the Ubuntu image on VM, go to Settings -> Shared Folders
5. Browse to the folder on your Mac you want to share (ubuntu-sf). Name the shared folder (ubuntu-sf)
6. On Ubuntu terminal, run : sudo adduser your-username vboxsf
7. On Ubuntu terminal run: sudo mount -t vboxsf ubuntu-sf /home/bparanj/ubuntu-sf
Here ubuntu-sf is the name of the shared folder we created in step 5. /home/bparanj/ubuntu-sf is the folder in the
Ubuntu.
References
How to share files between Virtual Box Ubuntu and host computer
How to automatically mount a folder and change ownership from root in virtualbox
