Restrict access to current members

I am new to PHP and MySQL and struggling with developing members-only access pages on the site for existing members of an organization. The organization I'm developing the site for provided an Excel spreadsheet with the details of the existing members. These people need access to more pages on the site than non-members visiting the site. I have been reading David Powers's books which have been tremendously helpful in dveloping my understanding of the technology. Example applications I've seen are for a registration system on a website that allows anyone to register on the site and have access to all pages. I need to allow full access to existing members only right off the bat. The second part of my quandary is how to set it up so that people can join the organization right on the site and then have immediate access to the full site. Any help is greatly appreciated!