The good news is that this works the same on either Red Hat/CentOS or Debian/Ubuntu: 

[donnie@localhost ~]$ sudo usermod -a -G marketing maggie
[sudo] password for donnie:

[donnie@localhost ~]$ groups maggie
maggie : maggie marketing
[donnie@localhost ~]$

In this case, -a wasn't necessary, because Maggie wasn't a member of any other secondary group. But, if she had already belonged to another group, -a would have been necessary to keep from overwriting any existing group information, thus removing her from the previous groups.

This method is especially handy for use on Ubuntu systems, where it is necessary to use adduser in order to create encrypted home directories. (As we saw in a previous chapter, adduser doesn't give you the chance to add a user to a group as you create the account.)