How to list all User Groups and Users
Something like this should work:

{% set userGroups = craft.userGroups.getAllGroups() %}

{% if userGroups|length %}
    {% for userGroup in userGroups %}
        &lt;h2&gt;
            {{ userGroup.name }}
        &lt;/h2&gt;

        {% set users = craft.users.group(userGroup.handle) %}

        &lt;ul&gt;
        {% for user in users %}
            &lt;li&gt;
                {{ user.username }}
            &lt;/li&gt;
        {% endfor %}
        &lt;/ul&gt;
    {% endfor %}
{% endif %}