How can you choose which group to output first when using the group filter? I am using the group filter to group like entries together. I need to be able to control which group is output first. How can I do this?
Rather than changing the order of the actual array returned by the group filter, you can just assemble a new array that has the keys in the order you want:

{# Get the grouped entries #}
{% groups = entries|group('foo') %}

{# Create an array of the group keys in the desired order #}
{% set keys = ['foo']|merge(groups|keys|without('foo')) %}

{# Now loop through the keys, rather than the groups directly #}
{% for key in keys %}

    {# Get the group with this key #}
    {% set group = groups[key] ?? [] %}

    {# Loop through the entries #}
    {% for entry in group %}
        ...
    {% endfor %}

{% endfor %}


(Note that the ?? operator requires Craft 2.6.)