Author List, Only Users With Entry
This should do it:

{# Grab all the entries for the section #}
{% set entries = craft.entries.section('news').find() %}

{# Group by the authorId #}
{% set entriesByAuthor = entries|group('authorId') %}

{# Grab all of the users with the userIds in entriesByAuthor #}
{% set authors = craft.users.id(entriesByAuthor|keys) %}

&lt;ul&gt;
    {% for author in authors %}
        &lt;li&gt;{{ author.username }}
    {%  endfor %}
&lt;/ul&gt;