Depending on your requirements that's quite easy to programm.

- Create a Module

- Register an Observer (Module/Name/etc/frontend/events.xml)

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_predispatch">
        <observer name="module_name_force_login_route_observer" instance="Module\Name\Observer\ControllerActionObserver"/>
    </event>
</config>

- And add an Observer with your custom logic redirecting to the login page.

<?php

namespace Module\Name\Observer;

use Magento\Framework\App\Response\Http;
use Magento\Framework\UrlFactory;
use Magento\Framework\App\Http\Context;
use Magento\Customer\Model\Context as CustomerContext;

class ControllerActionObserver implements ObserverInterface
{
    const LOGIN_CONTROLLER_ACTION = 'customer/account/login';

    protected $_response;
    protected $_urlFactory;
    protected $_context;

     public function __construct(
        Http $response,
        UrlFactory $urlFactory,
        Context $context
    ){
        $this->_response = $response;
        $this->_urlFactory = $urlFactory;
        $this->_context = $context;
    }

    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $request = $observer->getEvent()->getRequest();
        $loggedIn = $this->_context->getValue(CustomerContext::CONTEXT_AUTH);
        $group = $this->_context->getValue(CustomerContext::CONTEXT_GROUP);

        if ( //Some Logic ) {
            $url = $this->_urlFactory->create()->getUrl(self::LOGIN_CONTROLLER_ACTION)
            $this->_response->setRedirect($url);
        }
    }
}

As an example your logic could look like this to redirect all customers who are not logged in from the product view to the login page.

if (!loggedIn && strtolower($request->getFullActionName()) === 'catalog_product_view')