1. Make guest additions iso available on host server
VBoxManage storageattach "" --storagectl IDE --port 0 --device 0 --type dvddrive --medium /usr/share/virtualbox/VBoxGuestAdditions.iso

2. Mount cdrom and install/update guest additions in guest
sudo mount /dev/cdrom /media/cdrom
sudo bash /media/cdrom/VBoxLinuxAdditions.run
sudo umount /media/cdom

3. Reboot guest to ensure everything is loaded.
sudo reboot now

4. Remove CD iso
VBoxManage storageattach "" --storagectl IDE --port 0 --device 0 --medium "none"