The command line tools in Linux are very powerful, and as such come with a lot of options. For the most part, after the initial configuration of the system most of the options are not required except in ‘special’ circumstances. Here is an overview of what is available (in Fedora at least).

Create a new user

Add a user called 'auser' and add them into the secondary groups of 'apache' and 'sudo'

useradd -G apache,sudo auser

Create a new group

Create a new group in the system called 'agroup'

groupadd agroup

Set a users primary group

Set the primary group of 'auser' to 'agroup'

usermod -g agroup auser

Assign a user into an existing group

Add 'auser' into 'agroup' group

gpasswd -a auser agroup

Remove a user from a group

Remove 'auser' from 'agroup'

gpasswd -d auser agroup

Remove a user from the system

Remove 'auser' from the system and remove their home directory, mail spool file and name group if it exists

userdel -r auser