To add people to a group, you need to know their username (imagine that).
The command to add mark to the grads group would be:
$ sudo usermod -a -G grads mark
The -a flag stands for append (or add) and the -G flag specifies that they
should be added to the group name following.
Checking Groups
To check who is included in a certain group (we’re going to stick with the
grads example), then you would use:
$ grep grads /etc/group
Which will give you back the usernames of everyone in the group.
Removing Users from Groups
Well, if you can add people, it’s only logical you can remove them too.
To remove mark from the grads group, you would use:
$ sudo desluser mark grads
Warning: If you forget to include the group category in
this command, you will just… delete the user. That’s not good! Thus, a safer
way to do this would be through the “Users and Groups” window of the
GUI, which allows you to manage groups.