functions.php

// Automatically grant membership at registration
function sv_add_membership_at_registration( $user_id ) {
    $args = array(
        // Enter the ID (post ID) of the plan to grant at registration
        'plan_id'   => 253,
        'user_id'   => $user_id,
    );
    wc_memberships_create_user_membership( $args );
}
add_action( 'user_register', 'sv_add_membership_at_registration', 15 );