Want to add my custom prepare query but add_filter doesn’t run

The WP_User_Query allows meta_query searches exactly like the other WP_*_Query classes. Example here:

global $wpdb;
$author_search = new WP_User_Query( array(
    'role'       => 'subscriber', 
    'fields'     => 'all_with_meta',
    // if it's a digit/int/float, use 'meta_value_num'
    'orderby'    => 'meta_value',
    'order'      => 'ASC',
    // you could try -1 as well.
    'nuber'      => 99999999,
    'meta_query' => array(
        'key'     => 'my_userpoints',
        // only needed if you got a specific value you search for
        // 'value'   => '',
        // only needed if you got a specific value you search for
        // and want to compare against it. For e.g. '>', '<', etc. See Codex.
        // 'compare' => '',
        'type'    => 'CHAR', // this is the default, use INT if you got an int/float value
    ),
) );
$author_list   = $author_search->get_results();
$author_count  = $wpdb->num_rows;