I used to add a line to the /etc/sudoers file directly, using the 'visudo' command. However, I noticed that, as of Debian 6.0 "squeeze", the system is set up so that all users in the 'sudo' group have full access to sudo.
So, the problem arose - how to add a user to the 'sudo' group? For years, I used to add users to groups by simply editing /etc/group. Nowadays, "there's an app for that." OK, it's not quite an app, but the following command (or incantation, if you will) should add user "fuser" to the group "groupie":
# usermod -a -G groupie fuser
For more information, see "man usermod".