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Actually there is an easy way to do that:

1. Install the extension pack for VirtualBox.


2. Restart your virtual machine


3. 
   

   Install Guest Additions in your guest Ubuntu

   
   
   
   
   
   • You can mount the ISO which is on /media or press Left Control+D
     
   
   
   
   



4. Reboot



5. 
   

   Try to access /media/sf_your_shared_folder_name. If you still don't have access, that means you don't belong to the vboxsf group, as Nilo said. This command will solve your problem:

   
   
   

   sudo adduser your_username vboxsf
   
   

   
   



6. Log out and log in again to apply changes of adduser. See comments by kol and atcold below.



7. If you still can not see the shared folder, you have to mount it. You can activate automount for the shared folder in the options of VirtualBox Manager.


8. Restart again.

First, please make sure you have installed the Guest Additions

1. Start your VM




2. Devices > Insert Guest Additions CD image...
   



3. 
   

   Mount the CD:

   
   
   

   sudo mount /dev/cdrom /media/cdrom
   
   

   
   


4. 
   

   Install the necessary packages:

   
   
   

   sudo apt-get install make gcc linux-headers-$(uname -r)
   
   

   
   


5. 
   

   Install:

   
   
   

   sudo /media/cdrom/VBoxLinuxAdditions.run
   
   

   
   

Second, add your user to the group 'vboxsf':

~$ echo $USER; 

ahmed

~$ sudo usermod -a -G vboxsf ahmed

Reboot

Know that the label of your shared folder is lpi (for example):

Prepend sf_ to the label. Then, you will find your shared folder under /media/sf_lpi

Finally, you can also create a link to your home. For example:

ln -s /media/sf_lpi /home/ahmed/lpi

:)

Add the shared folder to the virtual machine using vBox graphical interface
Make sure to select automount and make permanent

Login to the virtual machine using a root account

Check vboxsf group exists

~$ grep vboxsf /etc/group

vboxsf:x:125:

Check user is not already in vboxsf group

~$ id nilo

uid=1000(nilo) gid=1000(nilo) groups=1000(nilo),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),109(lpadmin),124(sambashare)

Add user nilo to vboxsf group

~$ sudo usermod -a -G vboxsf nilo

Check again user groups

~$ id nilo

uid=1000(nilo) gid=1000(nilo) groups=1000(nilo),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),109(lpadmin),124(sambashare),125(vboxsf)

Reboot and login as nilo

Shared folder is now accesible in /media/sf_dropbox (dropbox is the name I gave to the share)

Share a folder between Host OS-> Windows and Guest OS ->Ubuntu(Virtual box)

Step 1
Install install Guest Additions from VirtualBox’s menu go to Devices->Install Guest Additions
This will mount a virtual CD on your /media/cdrom. As root user Open this /media/cdrom added folder using Open with terminal option(Right click with mouse).

Step 2
Run the program VBoxLinuxAdditions.run. When the program completes reboot your VirtualBox.

$ sudo ./VBoxLinuxAdditions.run

Step 3
Create a shared folder. From Virtual menu go to Devices->Shared Folders then add a new folder in the list, this folder should be the one in windows which you want to share with Ubuntu(Guest OS).
Make this created folder auto-mount.
Example -> Make a folder on Desktop with name Ubuntushare and add this folder.

Step 4
When done with you shared folder(s) specification, we mount folder from Ubuntu(Guest OS).
Create a mountpoint, this a directory in Ubuntu that will share files with the shared folder from Windows.
Run this to create a directory in Ubuntu

$ sudo mkdir ~/Desktop/windowsshare

Step 5
With your mountpoint created you can now mount the shared folder.
Run this command to share the folder:

$ sudo mount -t vboxsf Ubuntushare ~/Desktop/windowsshare

Ubuntushare is the name of folder we add in VirtualBox Devices section this folder is in Windows(Host OS).
~/Desktop/windowsshare is the directory in Ubuntu(Guest OS)

CONGRATULATIONS->
Now you can share the files between Windows and Ubuntu. Try adding any file in windows(Host OS) Ubuntu share folder now check Ubuntu(Guest OS) windowsshare directory the file will be reflected.

My Article

Shared folder b/w windows and ubuntu

1. Install the program called 'Programs and Group' from the Software Center


2. Choose the current user and click on Manage Groups


3. Choose vboxsf and click 'Add'


4. Reboot

1. Create a dir where you're going to mount this, like mkdir docs




2. Install Guest Additions into the guest Ubuntu machine.




3. Shutdown the Ubuntu guest, then configure it.




4. In Storage, setup your shared folder and make a note of what you set as the name in the second field. For example "crazy". Check the box for Auto-Mount.




5. Boot the Ubuntu guest.




6. Like you said, it won't actually be accessible. (This is the key part of your question.) You'll find instead it is present at /media/sf_crazy, where 'crazy' is the name you gave it. And, it will also be present in your home dir at ~/crazy.



7. 
   

   You now need to take one more step to actually mount it:

   
   
   

   sudo mount -o uid=1000,gid=1000 -t vboxsf ~/crazy ~/docs
   
   

   
   

Perform these steps after installing Guest Additions. For Windows 10 host environment, on the Windows machine I had to share folders.

1. Right click folder to share


2. Select "Share with"


3. Select "Specific People"


4. Chose Everyone, read/write options


5. On client Ubunto machine run command: sudo adduser xxxxxxx vboxsf

where xxxxxx is your user account name. Log out and log back in to Ubuntu.