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I tried searching but came up with nothing.
What you are looking at is a membership script. There are quite a few free and low cost solutions available over at www.hotscripts.com Its not quite a simple as creating a form, you need to have a membership database that contains all the data, and a way of preventing users simply accessing the pages.
Are you wanting to simply prevent users being able to access private sections of the site, or are you more looking towards delivering member based content, a bit like Yahoo do, they show a slightly different page depending on you being logged in or not.
Also, what sort of things do you want to offer members. A CMS script might be a good idea, this allows you to manage the entire site and also have control over members. A CMS is a content management system, again you will be able to find loads of these scripts at hotscripts.com
Hope this gives you some ideas.
A CMS would be nice too. I'll check the site you mentioned.
Thanks for the help.