1. useradd

If you do not wish to add new user account to into your Linux system by using a GUI tools, you can do this task from a command line with useradd command.

useradd -mc "user name" -s /bin/bash john

The previous command will create a new user account for user john. -mc options instruct a useradd comment to put a comment about new user which is very often use to specify user's full name. -s option will select path the a default shell.

2. adduser

Another way to add use is to use adduser command. However this command may not be available on every distribution of Linux. Simply type:

adduser john

and follow instructions to answer some details regarding to a new user account.