You can directly access the host computer’s shared folders from VirtualBox’s guest machines.
This is made possible by the 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 virtual machines.
To use shared folders, you must have the current version of VirtualBox Guest Additions. Installed on the guest operating system, you must configure your guest machine’s settings to specify which directories are to be shared.
With it, all you have to do is share the folder with the guest machine and easily access the folder content from the virtual machine.
Installing VirtualBox Guest Additions.
Before installing the 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 install 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, 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. Then, click Run to begin the installation.

After installing the tool, run the commands below to add your account to the VirtualBox file-sharing group.
sudo adduser USERNAME vboxsf
After that, shut down the Ubuntu virtual guest machine. Continue below to add the shared folder.
Share the Host Folder
Now that the guest tool is installed, edit the guest 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. Next, 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 the VirtualBox host folder with Ubuntu guest machines.
Congratulation! You successfully shared the VirtualBox host folder with the Ubuntu guest machine.
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”