openidp.org is a generic identity provider for individuals who do not have a Shibboleth-powered identity provider of their own. You are welcome and encouraged to set up your own identity provider, further information is available at the Shibboleth site.
openidp.org's original purpose was to provide access to myVocs: a collaboration environment. It is now available for general use.
You can register a new identity or manage an existing identity. Each identity requires a unique email address for password verification.
That's about it.
So you want to build a site like this?
Cool idea. ;)
Install Drupal. Customize it to your liking. If you want a strictly registration-only site like openidp.org, disable all features except account registration.
There is no integration between Drupal and Shib authentication. If you want that, all it requires is a good webSSO module for Drupal.