List of users inside custom taxonomy

get_terms and get_posts return arrays not objects.

Try this:

<?php $disciplines = get_terms('disciplines');
  foreach($disciplines as $discipline) {
    echo '<li><a href="#">' . $discipline['name'] . '</a></li>';

    $post_args = array( 'post_type'  =>  'users', 'disciplines' => $discipline['term_id']);
    $posts = get_posts( $post_args );

    foreach( $posts as $post ){
      echo 'post title: ' . $post['post_title'] . '<br />';
    }
  }

?>