The syntax of gpasswd is correct - assuming virtualbox is correct installed.
The group vboxusers does not exist on the guest - but the host as it is the user on the host which initiates USB passthrough.
$ groups
network scanner lp wheel manjaro
$ grep vbox /etc/group
vboxusers:x:108:
$ sudo gpasswd -a $USER vboxusers
Adding user manjaro to group vboxusers
The usermod command can used as well.
Two ways to achieve the same result but on the host.