I'm not sure this is the right place to ask, but I'll do my best to answer: first you need to create an account on the site. Once you have an account, you can ask the person who uploaded the private video to add you as a friend. If they add you as a friend, you are able to view all of their private videos.
*Most* members who have uploaded content will deny your request to be friends unless you have uploaded a good amount of content to the site yourself. They might even set your account to ignore, meaning they will not even see additional friendship requests from you. I recommend not asking people to be friends until you have contributed some of your own videos to the site. But most of all, read people's profiles before you send them a request, they often have guidelines on what is needed from you before they will accept your friend request and grant you access to their videos.