Some questions on the security model

Howdy all. I\'m working on a plugin or two and need some help with a few security concepts.

1) Is there a UID for a guest/anonymous?

2) Do I need to modify the access table directly to allow existing core groups access to the new features I am adding?

3) If I create new groups and I want, say, all registered users to be part of that group whats the best way of going about that?

There were some more really earthshattering questions and insights I\'ve had, but my masterful memory is failing me again so I\'ll come back to them.