Conditionally add free shipping to users based on their user role.

mindsize_wc_free_shipping_by_role.php

<?php

// Add free shipping for users in subscriber or administrator roles.

function mindsize_wc_conditional_free_shipping( $rates, $package ) {
	get_currentuserinfo();
	global $current_user;
	
	if ($current_user->ID) {
		$user_roles = $current_user->roles;
		$user_role = array_shift($user_roles);
		
		if ($user_role == 'subscriber' || $user_role == 'administrator') {
			$freeshipping = $rates['free_shipping'];
	 
			$rates = array();
			$rates[] = $freeshipping;
		} else {
			unset($rates['free_shipping']);
		}
	}
    
	return $rates;
}

add_filter( 'woocommerce_package_rates', 'mindsize_wc_conditional_free_shipping', 10, 2);