To add new users for the web interface of Nagios you will need to run the following command:
# htpasswd /usr/local/nagios/etc/htpasswd.users username01
Where "username01" is, type the name of the user you are trying to add. It will later prompt you for a password.
You will also need to edit your cgi configuration file.
Careful when editing the cgi config file. For security reasons you might not want to give full access to every user.