WordPress Customer Only Section

I need my website to have a customer only section, where current customers login and they are able to see pages and posts in the site that the general public can't see. I have a database of customers already and I need to connect the 2 databases or be able to sinc the databases on a regular basis. This should be an automated or very easy process. Here is the current site. [login to view URL] In your proposal, include your idea of how to connect or sinc the 2 databases. Please read the project first and write the word "Understood". Also write your steps and suggestions to complete the project with a short description of what you understood.