Basics of overriding an admin page template - Craft 3
There are multiple ways to achieve your goal, I think the most elegant one is to overwrite the default route /users with your custom one and replace the elementType

Things you need to do 


Change the default route to your template
Copy the default template and change it's element type
Create a custom element type that extends craft/elements/User


You can change the route of all navigation  bar items (modifyCpNav)

Event::on(
    Cp::class, 
    Cp::EVENT_REGISTER_CP_NAV_ITEMS, 
    function(RegisterCpNavItemsEvent $event){
        // the 4th element is the "User", you can change the URL to your
        // custom template instead of the default template
        // I suggest you to loop all `navItems` and find 
        // the correct user instead of assuming it's the 4th
        // Craft::dd($event-&gt;navItems);
        $event-&gt;navItems[4]['url'] = 'path/to/template/user';
});


Then just copy the Craft templates/user/index template in your plugin to create an index page

{% if CraftEdition != CraftPro %}
    {% exit 404 %}
{% endif %}

{% requirePermission 'editUsers' %}

{% extends "_layouts/elementindex" %}
{% set title = "Users"|t('app') %}
// changes start
// replace the elementType with your custom element type
{% set elementType = 'prefix\\namespace\\elements\\User' %}
// changes end
{% block actionButton %}
    {% if currentUser.can('registerUsers') %}
        &lt;a class="btn submit add icon" href="{{ url('users/new') }}"&gt;{{ "New user"|t('app') }}&lt;/a&gt;
    {% endif %}
{% endblock %}


Then in your src/elements folder create a new User class to overwrite the sources

class User extends \craft\elements\User{ //&lt;-- extend the default User element

    /**
     * @inheritdoc
     */
    protected static function defineSources(string $context = null): array
    {
        $sources = [
            [
                'key' =&gt; '*',
                'label' =&gt; Craft::t('app', 'All users'),
                'criteria' =&gt; ['status' =&gt; null],
                'hasThumbs' =&gt; true
            ]
        ];

        // Overwrite the sources / usergroups...
        // usually Craft will search for usergroups at this place..
        // just leave it empty, so you'll only see the "default" group

        return $sources;
    }
}


Don't forget to register your element

TL/DR
You could as well just hide the other sources via CSS/Javascript - but I would not suggest it, since everyone is able to see the other groups with minor Javascript skills