By
{% assign last_pos=include.authors.size | minus: 1 %}
{% assign and_pos=include.authors.size | minus: 2 %}
{% for i in (0..last_pos) %}
  {% assign author_id=include.authors[i] %}
  {% assign author=site.authors[author_id] %}
  {% if author %}
    <a href="mailto:{{ author.email }}">{{ author.name }}</a>
  {% else %}
    {{ author_id }}
  {% endif %}
  {% if i == and_pos %}
    and
  {% elsif i < and_pos %}
    ,
  {% endif %}
{% endfor %}
on <span class="text-muted">{{ include.date | date_to_long_string }}</span>