I was digging around searching on how to install the VirtualBox Guest additions without the GUI using the command line. Most of the tutorials out there were of no help. Either with bad or outdated information. I found one article on xmodulo that worked for me.
Here's the steps I followed for my Ubuntu Server VM.
First downloaded the vbox guest addition. Replace X.X.X with your version. You can find that by going to the about within your virtual box install.
Then install the packages that is requires.
sudo apt-get install dkms gcc
sudo mount -o loop VBoxGuestAdditions\_4.2.16.iso /mnt cd /mnt
Visit the Source for additional information and how to on other distros.