How do I grant sudo privileges to an existing user?

You need to add the user hduser to the sudo group (which is the "administrators" group in Ubuntu).

If you have already created the user, you can add the user to the sudo group by running the following command in a Terminal:

sudo usermod -a -G sudo hduser

Instead you can try,

sudo adduser hduser sudo

In Ubuntu you need to add the user only to the group sudo.

1) Become root. You can do this using sudo -i or becoming root the old fashioned way su -

2) Run visudo

3) I changed this portion of the sudoers file to have my chosen users become sudo users, and you can add users similarly (blank lines introduce to format cleanly):

## User Aliases
## These aren't often necessary, as you can use regular groups
## (ie, from files, LDAP, NIS, etc) in this file - just use %groupname
## rather than USERALIAS
# User_Alias ADMINS = jsmith, mikem dbadmin 
ALL=(ALL) ALL
ics ALL=(ALL) ALL 
csm ALL=(ALL) ALL 
coa ALL=(ALL) ALL

4) Press : and x to write the changes to sudoers and exit vi.