Can we add a Register option on our WP Blogs?

I just wanted to ask, is it possible to add a registration area to my blog? I have heard that it is possible to have people subscribe, but can members of one of my forums, actually become members of my blog? If they can, how do I add new members, I can not find any registration thing, and I really don’t know if that is possible.

[mod note: snipped – hopefully irrelevant now]