VirtualBox will fail to install the correct dependencies when running VboxLinuxAdditions.run… here is a quick fix. See the debian wiki for additional details. These specific instructions are tailored for Debian 8.
apt-get install virtualbox-guest-dkms build-essential linux-headers-$(uname -r)
Then you can run
Reboot / Restart services and you’re good to go!
If you need to check that the guest additions are available,
systemctl status virtualbox-guest-utils.service && lsmod | grep vbox
Should check that the guest service is running, and also check for vbox modules.