how did you get this output "useradd: user blairellis exists" since its to add a user to a group and not make a new user.
so you should of typed in:
sudo useradd -G vboxusers blairellis
If you did that and it gave that error than idk.
Do you happen to now what kernel driver it claims it needs?
If you know it you can try to load it.
sudo modprobe kernel_module