You can use add role function like
<?php add_role( $role, $display_name, $capabilities ); ?>
Example
add_role('basic_contributor', 'Basic Contributor', array(
'read' => true, // True allows that capability
'edit_posts' => true,
'delete_posts' => false, // Use false to explicitly deny
));
Also see this tutorial (custom rule discussed) and this plugin too if you don't want to write any code.