S2member is not really designed to work this way out of the box. It is assuming that the user will be creating an account via wordpress or a thirdparty software in the wp database.
However, you could use simple php conditionals on your pages (or even make your own shortcode to do this) using the s2member constant S2MEMBER_CURRENT_USER_IP.
Something like this maybe:
<?php
//IP Ranges
$lowest_ip = '100.100.5.0';
$highest_ip = '100.100.5.100';
if(S2MEMBER_CURRENT_USER_IP >= $lowest_ip && S2MEMBER_CURRENT_USER_IP <= $highest_ip){
//Show the info
echo ' ';
}
else {
//Show something else like an error
echo ' ';
}
?>
NOTE: if you are adding this to a page, you will need to enable a php execution plugin like this:
http://wordpress.org/extend/plugins/php-execution-plugin/
Hope that helps
Dave