add_action ( 'wpmu_activate_user', array( $this, 'assign_user_to_blog') );
public function assign_user_to_blog($user_id, $password, $meta ) {

    if ( isset($meta['role']) ) {
            add_user_to_blog( '3', $user_id, $meta['role']);

    } else {
            add_user_to_blog( '3', $user_id, "dmd");

    }

}

add_filter( 'add_signup_meta', array($this, 'add_register_meta' ) );
public function add_register_meta($meta = array()) {

    $role = sanitize_text_field( $_POST['role'] );
    $meta['role'] =  $role;

    return $meta;

}