what's wrong with my command in usermod?

[Gabriel M Dragffy]

Looking at the help output/man pages of usermod, I find this:

 -G, --groups GROUPS           list of supplementary groups
                -a, --append    Use -a option to append the user

So the -a should append said user to the list of groups. If I attemp the
following command I get an unexpected result:

$ sudo usermod -a nogroup username
Password:
Usage: usermod [options] LOGIN

And it just continues to list the help pages. Hmmm! I guess I'm reading
it wrong, but which part?

Cheers,
Gabe