Let's call the site I need [login to view URL], a site where you can store your sites, usernames and passwords so you can enter them with a single click.
Here's how it should work:
User registers
User gets email to confirm registration
Once logged in, user enters urls for other sites where they are members
User also enters user names and passwords for these sites
User sees list of their registered sites as links
Clicking the link loads that particular site in logged-in state
I will supply wireframe and more specific requirements document.