The first user created on the machine already can do everything (with sudo, or gksu, or sudo su) that a user may ever need to do as root. If a task really needs you to be root, you can temporarily become "root" with "sudo su" command. Follow the link provided by prodigy to see further details on this. I'm posting here some common use examples -
Originally Posted by nrmurali
Adding an Existing User to an Existing Group :
To grant users the ability to do things as root, simply add them to "sudo" group. For example, to add a user "newuser" to this group -
(the first 'sudo' is there to grant you the root permission, as this task needs that. The command part is adduser newuser sudo)
sudo adduser newuser sudo
To grant this user all the same privileges as the first user, check all the groups that the first user is a member of, then make the new user member of all those groups just like above. For example, if the first user (created while installation) is "firstuser", then,
To see all the groups that "firstuser" is member of -
Output will be something like -
..means user "firstuser" is a member of a total of 10 groups - "firstuser, adm, disk,........., vboxusers".
firstuser adm disk cdrom sudo dip plugdev lpadmin sambashare vboxusers
Now, to grant "newuser" the membership of "adm" group -
Removing a user from a group :
sudo adduser newuser adm
Let's say you added the user "newuser" to "adm" group by mistake, or for some other reason you want to remove him from that group. Simply do -
..this ^^ will remove the user "newuser" from the "adm" group.
sudo deluser newuser adm
For more info on manipulating users & groups, see the manpages of these commads using -
Now something you should know and ALWAYS keep in mind: The "root" is disabled by default in Ubuntu for a good reason. Enabling and logging in as root or working with root privileges All the Time is almost never required. If you do so without a good reason and without understanding the possible consequences (of a wrong action), you may be shooting yourself in the foot, or maybe in the head!! That being said, some more info -
The "root" is a member of its own group - "root". As such, adding a user to this group *should* grant them the same powers as root, but I can't say for sure because I never tried that (if you are experimenting on the current user, you may have to log-out -> login again for the change to take effect). Despite that being fairly easy to do, just like the above examples, it is recommended to NOT DO SO. Almost everything you may ever need to do as root can be done with sudo, so use that group instead.
Hope it answers your question as well as serves as a general advice to anyone interested.