[dtgmap]

I finally figured this out after combining info from these 2 posts:

http://forum.x-cart.com/showthread.php?t=9081&highlight=shippingid
http://forum.x-cart.com/showthread.php?t=60983

1) Create an entry in xcart_config that has the following values:
name: free_shipping_allowed_method
comment: FREE SHIPPING ALLOWED METHOD
Give free shipping on a certain shipping method (Example: FedEx Ground is 43)
value: 0
category: Shipping
orderby: 4
type: text
defvalue: 0

2) In include/func/func.cart.php, insert this code:

if ($config['Shipping']['free_shipping_allowed_method'] == $shipping_id && $user_account['membership'] == 'Friends'){
                $shipping_cost = 0;
}

BEFORE

    $display_shipping_cost = $shipping_cost;

    if (
        $calculate_enable_flag
        && !(
            @$customer_info['tax_exempt'] == 'Y'
            && (
                $config['Taxes']['enable_user_tax_exemption'] == 'Y'
                || defined('XAOM')
            )
        )
    ) {

        // Calculate taxes cost

3) Go into the x-cart admin, under Shipping Options, and set your shipping method
(This number will be the shippingid in the xcart_shipping table)

If you have any questions, there is valuable info in the 2 posts I referenced, such as applying to multiple shipping methods.

Note: This code change may affect your coupons. I don't currently have any set up, so I haven't tested it yet, but one of the posts addresses this scenario.