A sample CGI configuration file (/usr/local/nagios/etc/cgi.cfg) is installed by default.

If you still have some permission issue:
1. edit your cgi.cfg and add the users to the lines where “nagiosadmin” is, not all of them, it depend of the right you want to give to the user

Code:
authorized_for_system_information=nagiosadmin,user1,user2,...usern

2. Create the user and the password for the user you just add to your config file whithout the -c.

Code:
htpasswd /usr/local/nagios/etc/htpasswd.users user2

3. Restart apache.

Code:
/etc/init.d/apache2 restart