How do I create an administrator user on Ubuntu?

First, create the user with:

sudo adduser <username>

You can read more about this command in the man pages of your system with man adduser.

You can then add a user to the sudo group with with the command:

sudo adduser <username> sudo

Note that versions of Ubuntu until 11.10 will use admin as group instead of sudo:

Until Ubuntu 11.10, the Unix group for administrators with root privileges through sudo had been admin. Starting with Ubuntu 12.04 LTS, it is now sudo, for compatibility with Debian and sudo itself. However, for backwards compatibility, admin group members are still recognized as administrators

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If your system does not, then we need to mess with the sudoers file to grant sudo permissions. You can read about the sudoers file with man sudoers for details on the exact syntax and available options, but for simplicity's sake, you can do either of the following:

• Create a group with the addgroup command, and then add that group to the sudoers file. Use addgroup <groupname> to create the group, and then edit the sudoers file (sudo visudo) and add the line %<groupname> ALL=(ALL) ALL to the bottom
• Edit the sudoers file with sudo visudo, and add <username> ALL=(ALL) ALL at the bottom for each user you want to add.

The "popular" answer is how to "reimplement", not "how to add the user?". Bare minimum you need to do is this:

usermod -a -G sudo USERNAME

On my particular system, I am a member of the following groups:

usermod -a -G adm,cdrom,sudo,dip,plugdev,lpadmin,sambashare,libvirtd USERNAME

To verify what you have done:

groups USERNAME

Choose System -> Administration -> Users and Groups.

Select Add to add your new user. When you have completed the wizard, choose your new user and click on account type and change from Desktop user to Administrator.