First create a file which holds username and password.
For this you can use htpasswd program.
/usr/local/httpd/bin/htpasswd -c /usr/local/httpd/passwordfile username
The -c option here creates new file. So if the file already exists or you are just modifying or adding a new user omit -c option. Not doing that will make a new file resulting in loss of old username and password.
The file must be somewhere your webserver can find but not other place. Though the password is encrypted its better to keep it off from human eye.
Now edit you httpd.conf file to include the follwoing line in whichever websites that you require authentication
AuthName "By Invitation Only"
Require user rbowen sungo
Restart apache and you're done.
Additional reading: http://httpd.apache.org/docs/1.3/howto/auth.html#basic