How to change group by cmmand

Hi,

I have problem to change group name on my virtual host. I can do that of course by Virtualmin interface (Virtualmin - Server configuration - Change domain name - Change user name suffix and group) but it will be nice to do that by command line. With such command script could change group it self without playing with that manually entering website. Does anyone know how to do it? What is the command line that I can be use? I tried "virtualmin modify-user --domain domain.name --user username --add-group groupname" but it doesn’t work.

Kind regards