Here is a basic template that requires login, and checks that the currently logged-in user is in the user group 'clients'.

    {% extends "_layout" %}
    {% requireLogin %}    

    {% block content %}
    
        {% if currentUser.isInGroup( craft.userGroups.getGroupByHandle('clients') ) %}
	
            {# Only viewable by logged in users that belong to 'clients' user group #}    
            Restricted client content goes here.

        {% else %}
            
            {# Viewable by logged in users not in the user group 'clients' #}
            Notice to other user groups that this content is restricted to clients only.
    
	    {% endif %}

    {% endblock %}

You may also want to check out [this question][1] for more examples.

  [1]: http://craftcms.stackexchange.com/questions/4182/password-protected-area-with-unique-files/4184#4184