Count all unique values
Something like this should do the trick:

{% set usersGroupedByCountry = craft.users.limit(null)|group('postaddress.country') %}

&lt;ul&gt;
    {% for country, users in usersGroupedByCountry %}
        {% if country and users|length %}
           &lt;li&gt;{{ country }} ({{ users|length }})&lt;/li&gt;
        {% endif %}   
    {% endfor %}
&lt;/ul&gt;


Note that if you want to order the list of countries, you'll need the SuperSort plugin. Here's how you would sort the list alphabetically:

{% set usersGroupedByCountry = craft.users.limit(null)|group('postaddress.country)|supersort('ksort') %}


Also note that if you have a lot of users (say, hundreds), it is probably a good idea to wrap the whole thing (including the {% set usersGroupedByCountry ... %} part) in a {% cache %} tag pair, for performance reasons.