Grouping Super Table Results via Twig Multidimensional Array
For grouping arrays, use Craft's group filter or index filter, then for joining the array items with commas use Twig's join filter.
Something like this might be what you're looking for (I've also added eager loading for improved performance, and I used the map filter as a cleaner way of organising the array, though you could use a nested loop if you find it easier to read):
{% set featuredPeopleGrouped = book.contributors
    .featured(1)
    .with(['contribution','contributor'])
    .all()|group('contribution[0]')
%}

&lt;dl&gt;
    {% for contributionType, rowData in featuredPeopleGrouped %}
        {% set contributors = rowData|map(row =&gt; row.contributor[0].title) %}
        &lt;dt&gt;{{ contributionType }}: &lt;/dt&gt;
        &lt;dd&gt;{{ contributors|join(', ') }}&lt;/dd&gt;
    {% endfor %}
&lt;/dl&gt;