I say:
$ VBoxManage controlvm <vbox uuid> usbattach <device uuid>
VirtualBox says:
VBoxManage: error: USB device '<device>' with UUID <uuid> is being exclusively used by the host computer
I say:
$ gpasswd -a <vbox user> plugdev
and log out the vbox user completely (no vms running, no logged-in shell).
And now the device is available and add-able.