Mac am I to understand only those who are registered to my blog/site can view the stats?
Yes, as long as they are not Subscribers, which is the default user role. They would have to manually promoted by you to a higher role in order to view your stats.
As for logged in I don't quite follow you.
If you required your visitors to register and login to comment (an ufortunately common and annoying anti-spam practice, just my opinion), they would of course have to login to comment. Therefore, any visitor wishing to comment on your blog would be logged in and untracked.
I have wp admin bar installed. Is that visible to visitors?
It's only visible to logged in users with the Administrator role.
what about those who have blogs at wp .com? could they use the admin bar to login?
WordPress.com and your self-hosted WordPress blog are entirely separate.