/usr/bin/VBoxClient and /usr/bin/VBoxControl must have SUID-bit otherwise it doesn't start and functionality doesn't enabled (seamless windows, autoresize, clipboard & etc.) Reproducible: Always Steps to Reproduce:
That's what the "vboxguest" group is used for. Add your users to that group, relogin the users and both programs should work. I just looked at the virtualbox-guest-additions and recognized that they lack an accordingly elog message. I gonna fix that.