Getting shared folder to work with a virtual ubuntu in Virtualbox is a bit of a nightmare.
Initially they simply can’t be accessed, even after re-installing Virtualbox guest add-ons.
The reason and solution is described here, but adding root account to group vboxsf didn’t work as well because of this problem: "usermod: cannot lock /etc/passwd; try again later.".
Thus, to summarize this mess, to make it work
# you have to reboot into recovery mode
# run this: mount -o remount,rw /
# then that: usermod -a -G vboxsf adminuser
# resume booting into normal mode
Shared folders BTW can be found under /media.