Use this function along with the Gravity Forms User Registration add-on to add users to an RCP subscription upon registration.
function iwitness_gf_rcp_user_registration( $user_id, $feed, $entry ) {
// retrieve subscription id from radio input
$subscription_id = rgar( $entry, '5' );
// create customer from $user_id
$customer_id = rcp_add_customer( array(
'user_id' => $user_id
) );
// add membership to new customer
rcp_add_membership( array(
'customer_id' => $customer_id,
'object_id' => $subscription_id,
'status' => 'active'
) );
delete_user_meta( $user_id, 'rcp_signup_subscription_id' );
}
add_action( 'gform_user_registered', 'iwitness_gf_rcp_user_registration', 10, 3 );