Bulk "Add Role" Action on the Users Screen

add_filter( 'bulk_actions-users', function ( $actions ) {
    if ( ! current_user_can( 'promote_users' ) ) {
        return $actions;
    }

    foreach ( get_editable_roles() as $role => $details ) {
        if ( 'administrator' !== $role ) {
            $actions[ 'wpide_add_role_' . $role ] = sprintf( 'Add role: %s', translate_user_role( $details['name'] ) );
        }
    }

    return $actions;
} );

add_filter( 'handle_bulk_actions-users', function ( $redirect, $action, $user_ids ) {
    if ( 0 !== strpos( $action, 'wpide_add_role_' ) ) {
        return $redirect;
    }

    $role = substr( $action, 15 );

    if ( 'administrator' === $role || ! array_key_exists( $role, get_editable_roles() ) ) {
        return $redirect;
    }

    $done = 0;

    foreach ( array_map( 'intval', (array) $user_ids ) as $user_id ) {
        if ( current_user_can( 'promote_user', $user_id ) ) {
            ( new WP_User( $user_id ) )->add_role( $role );
            $done++;
        }
    }

    return add_query_arg( 'wpide_roles_added', $done, $redirect );
}, 10, 3 );

add_action( 'admin_notices', function () {
    if ( isset( $_GET['wpide_roles_added'] ) ) {
        $count = absint( $_GET['wpide_roles_added'] );
        echo '<div class="notice notice-success is-dismissible"><p>' . esc_html( sprintf( _n( 'Role added to %d user.', 'Role added to %d users.', $count ), $count ) ) . '</p></div>';
    }
} );