How can I use a where statement with grouped results?
This should work:

{% for date, attempt in attempts|group('attemptDate') %}
    &lt;tr&gt;
      &lt;td&gt;{{ date|date('M j, Y') }}&lt;/td&gt;
      {% for entry in attempt %}
        {% if entry.type == 'My Type' %}
          &lt;td&gt;{{ entry.title }}&lt;/td&gt;
        {% endif %}
      {% endfor %}
    &lt;/tr&gt;
  {% endfor %}


I don't know what you want to do with the 'type' of the entries?
But if you want to limit the amount of entries according to type, you could fill "attempts" with all entries of a certain type first and then loop through them. Than you could remove the if statement which would be cleaner and faster I think.

So something like: 

{% set attempts = craft.entries.section("something").type("My Type").find() %}