BuddyPress.org

If a member has not logged in at least once, they are not considered ‘active’ and will not show in the count.

You could run this once in your theme/functions.php and then remove it. It will create an activity timestamp for all members with the role: subscriber.

function pp_add_last_activity() {

  $subscribers =  get_users( 'fields=ID&role=subscriber' );
  
  foreach ( $subscribers as $user_id ) {
        bp_update_user_last_activity( $user_id, bp_core_current_time() );
  }

}
add_action('bp_init', 'pp_add_last_activity' );