Access VirtualBox Host Folders from Ubuntu 17.10 Guest Machines
From VirtualBox’s guest machines you can directly access the host computer shared folders. This is made possible by VirtualBox host folder sharing feature. With shared folders, you can easily share files among virtual machines and the host computer. Folder sharing exposes the host computer files and folders to the virtual machines.
To use shared folders, you must have the current version of VirtualBox Guest Additions… installed on the guest operating system and you must configure your guest machines settings to specify which directories are to be shared.
This brief tutorial shows students and new users how to enable VirtualBox host folder sharing so guest machines can directly access folders shared on the host computer. Without this folder sharing feature, it would be very difficult getting huge files or folders to the guest machine.
With it, all you have to do is share the folder to the guest machine and from the virtual machine access the folder content easily.
Step 1: Installing VirtualBox Guest Additions…
Before installing VirtualBox guest additions tool, run the commands below on the Ubuntu guest machine first.
sudo apt install linux-headers-$(uname -r) build-essential dkms
After installing the packages above, restart and continue installing the Guest Additions Tool…
To get shared folders working, you must first install VirtualBox guest additions… on the guest machines. For this tutorial we’re using Ubuntu 17.10 guest machine. To install the tool, turn the guest machine on, then select the Devices and click Insert Guest Additions CD image…

This should mount a virtual CD/DVD drive on the Ubuntu guest machine and prompt you to run the installation. Click Run to begin the installation.

After installing the tool run the commands below to add your account to VirtualBox file sharing group…
sudo adduser USERNAME vboxsf
After that, shutdown the Ubuntu virtual guest machine. Continue below to add the shared folder
Step 2: Share the Host Folder
Now that the guest tool is installed, edit the gust machine settings to add the folder you want to share with the host machine. Then select the Settings tab and pick the Shared Folders from the list. Then enable it and locate the folder you wish to add. Click OK when done.

Save your changes and exit. Turn on the Ubuntu machine and browse to the /media/sf_Download folder and there you should see the shared host folder mounted on the Ubuntu guest machine.

This is how to share VirtualBox host folder with Ubuntu guest machines.
Congratulation! You’ve just successfully shared VirtualBox host folder with Ubuntu guest machine.
You may also like the post below:
This is not going to work, because of this:
https://forums.virtualbox.org/viewtopic.php?f=3&t=85186&sid=f869ff81e42faa1f433b1f725c952b8c&start=45
Thanks so much, I was searching for this!
How to fix (credit: bignumber9):
1. Install the guest OS as usual.
2. apt-get install linux-headers-$(uname -r)
3. apt-get install build-essential
4. apt-get install dkms
5. Reboot
6. Install guest additions as usual.
This blog post is fake, but at least I found my answer through it.
Thanks, I have updated the post.
You need to install a Perl compiler, first:
https://help.ubuntu.com/community/InstallingCompilers
This method works and it’s the simplest, permanent solution I found. Thanks
The “sudo adduser USERNAME vboxsf” should be “sudo adduser $USERNAME vboxsf”