How to setup VirtualBox Guest Additions Shared Folder on Ubuntu Server

Install Ubuntu Server in VirtualBox (with out any additional packages, may change depending on your need)

Create a Shared folder on your VM's settings.

Setup the build enviorment in the VM:
sudo apt-get build-essentials linux-headers-generic

Insert the CD image:
Devices->Insert Guest Additions CD image

It will be mounted at /dev/cdrom (most likely). If not identify the image using:
sudo blkid

Mount it to a directory in your system so that you can access the contents
sudo mount <source> <target>

Go to the mounted directory and Install the Guest Additions:
sudo sh ./VBoxLinuxAdditions.run

Reboot your system

Add an entry in rc.local for automatic mount:
sudo vi /etc/rc.local

And make the below entry

mount.vboxsf <share_name> <target_directory> vboxsf

Now you may create a shortcut to the target

ln -s <target_directory> /home/<username>/<sharename>

Reboot the system, you should be able to access your share using the target_directory or the symbolic link