Rewrite you catalog/product/view controller and add that logic.
Rewrite Block of that page and show error message accordingly.
Any other..?

Yeha, I do have another option as well. The Event-Observer Method. You can observe an event controller_action_predispatch_catalog_product_view and set up an observer where you can write your own logic there and redirect accordingly.

You can observe that event by setting up your config something like this.

<controller_action_predispatch_catalog_product_view>
	<observers>
		<mymodel>
			<type>singleton</type>
			<class>mymodel/controller_observer</class>
			<method>controller_action_predispatch_catalog_product_view</method>
		</mymodel>
	</observers>
</controller_action_predispatch_catalog_product_view>

And on the observer’s controller_action_predispatch_catalog_product_view method, you can check your logic for visibiliy of that product to the logged in user group and redirect if not visible.

Here’s the main point of this post (Did I take a lot of your time ?), in that same method you can directly redirect using the following snippet of code.

Mage::app()->getResponse()->setRedirect(Mage::getUrl("myrouter/mycontroller/noview"));

Adding an error message to the session would be a good idea, for user to understand what is happening. Else you can directly call a template in the redirected controller’s action with appropriate message.

I think this sums up the post for now..and as always hoping it helps somebody and waiting for the response.

[UPDATE]
New Module in Magento connect out to help you working in Ajax.

Let’s first know in Magento terms what we need.

Controller: The url on which Ajax will work on, or the request URL. You will have to set up the controller with its frontend router set in config.xml and its corresponding controller class.
Layout: Layout to handle the requested URL and return HTML if required.
Block: The block to call through layout for the above controller.

Now lets be descriptive.

If you want to show a loader while request is being processed just add the following whereever you like in the phtml page.First fetch the Javascript.

<script src="<?php echo $this->getJsUrl() ?>mage/adminhtml/loader.js" type="text/javascript"></script>

Then echo the loader which pops up. This loader will be similar to that of the admin.

<div id="loadingmask" style="display: none;">
<div class="loader" id="loading-mask-loader"><img src="<?php echo str_replace("index.php/","",$this->getUrl()) ?>skin/adminhtml/default/default/images/ajax-loader-tr.gif" alt="<?php echo $this->__('Loading...') ?>"/><?php echo $this->__('Loading...') ?></div>
<div id="loading-mask"></div>
</div>

Now to Call Ajax, use the following lines

/*Please note that the URL is created in reloadurl. Also see that the response text will be echoed in div with id=output-div*/

var reloadurl = '<?php echo $this->getUrl('router/controller/action') ?>';
Element.show('loadingmask');
new Ajax.Request(reloadurl, {
method: 'post',
parameters: "Params_Here",
onComplete: function(transport) {
Element.hide('loadingmask');
$('output-div').innerHTML = "";
$('output-div').innerHTML = transport.responseText;

}
});

After the Ajax Request is made it goes to your controller’s action, which in turn sees to your layout as follows:

class Namespace_module_frontendController extends Mage_Core_Controller_Front_Action
{
public function actionAction(){
$this->loadLayout()->renderLayout();
}
}

And here is the main part in layout where I spent most of my time in. Since in layout we will have to define reference where the html will echo (Most of the times), i got stuck here, because i need to echo the output on the div with id output-div not in any reference.And the trick is to name the layout as root and output as html. Like the following:

<module_controller_action>
<block type="module/block"  name="root" output="toHtml"  template="module/template.phtml"/>
</module_controller_action>

You are done now! What ever you write or echo in phtml file you will see populated in the DIV. Now you can treat block as normally as you do before.

Happy Coding!!