Assuming your Ubuntu guest username is USERNAME:
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Make sure you have already installed the VirtualBox Guest Additions in the Ubuntu guest machine
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From the Ubuntu guest:
sudo adduser USERNAME vboxsf -
Using the VirtualBox UI (Devices->Shared Folder Settings…) add the folder in the hosting machine that you wish to share.
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Restart the Ubuntu guest! (wasted too much time on that…)
ls /media/– you’ll find your shared folder there. You might wish to mount it to a different directory on your Ubuntu guest