Build something that works
These pop-ups are called modals. If you're using Bootstrap, you can read more about how to set them up in their documentation. Here's the link:
If you're just creating an informational modal, you should be able to just follow the Bootstrap instructions. However, if you want to use a modal containing a form, you'll likely need to use something called AJAX to submit the form. AJAX allows you to submit a form without redirecting to another URL - in other words, you can submit the sign-up form and have the modal close and now display the content for registered users. Setting up AJAX and modals may be a bit complicated if you're new to programming, but I highly recommend that you do some Google searching to learn more about it as it's a very useful technique!