//You can use the following code inside the child theme, it will add a timestamp for all users and remove code once all member starts displaying in your member list.

function buddypress_add_last_activity() {
  $members =  get_users( 'fields=ID' );
  foreach ( $members as $user_id ) {
        bp_update_user_last_activity( $user_id, bp_core_current_time() );
  }

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