Sudo is a program that allows users to run programs with the security privileges of another user (normally the superuser, or root). The /etc/sudoers file controls who can run what commands as what users on what machines and can also control special things such as whether you need a password for particular commands.
To add user to sudoers file, execute this command:
$ sudo visudo
And create new line at the bottom of file:
pikachu ALL=(ALL) ALL
Above line will allow user pikachu to run any command on any host as any user.
The structure is HOST=(USER) COMMAND
Another configuration example:
User_Alias OPERATORS = joe, mike, jude
Runas_Alias OP = root, operator
Host_Alias OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm
# The users in the OPERATORS group can run any command from any terminal.
linus ALL=(OP) ALL
# The user linus can run any command from any terminal as any user
in the OP group (root or operator).
user2 OFNET=(ALL) ALL
# user user2 may run any command from any machine in the OFNET network,
as any user.
user3 ALL= PRINTING
# user user3 may run lpc and lprm from any machine.
go2linux ALL=(ALL) ALL
# user go2linux may run any command from any machine acting as any user.
Reference: ubuntu.com, go2linux.org