How to add & remove users by command line in ubuntu?

To add a user you must use be root or use the sudo command. Here are the commands:

To add a user. NOTE: do not use the useradd command.

$ sudo adduser <username>

To see the options for adding a user try the man command.

$ man adduser

Add user

Here is a useful example of the useradd command. Why use useradd? It gives a few more options for special cases. To add a user, give her a standard home directory in the /home folder and specify the shell she accesses by default do this:

$ sudo useradd username -m -s /bin/bash 
$ sudo passwd username 

Groups

You might also wish to create a new group for your users.

# sudo addgroup <groupname>

To add a new user to a existing group you would do this:

# sudo adduser <username> audio

To add an existing user to an existing group you can use the same command:

# sudo adduser <username> <groupname>

or

# sudo usermod -aG <groupname> <username>

Delete  user

To remove a user from a system without deleting the users home directory.

# sudo userdel username

To remove the user and delete the users home folder:

# sudo userdel -r username