When you add a linux user it gets it's own group so say the user is called "bob" it will be in the group "bob" no need to add it to another group.
To add it to sudo use the
Code:
visudo
command and add
Code:
bob ALL=(ALL) ALL
Under the bit where it says root.
As for dokuwiki you're opening a whole can-o-worms on what permissions and groups you should assign web stuff.
Personally I make a user for each of my sites and store the files in /home/<user>/public_html, the files are then owned and grouped by to <user>, with 0644 permissions on files and 0755 on directories (owner writable, everything else readable).
Some people prefer to store them in the /var/www folder, what you don't want to do is allow the files to be writable by the web server, so don't chown www-data or whatever your webserver runs as.