When you make a page private you can only assign one password for anyone to access that, and only that, page. This method is great for when you want to hide a single page's content, though, don't need nor want to create users in your system.
In your case, however, you really want to create a new user for each person I would assume at subscriber level, or, allow them to register on their own.
From this point, there are multiple ways to hide content from non-logged in users. If you are not good with php you can try a plugin such as this one. (I have not used this plugin, I am suggesting you find one that works best for you)
Also, now would be a good time to make sure your Settings/General are correct for registration.