Contributors must be invited by blog owner/moderator. That's true of ALL blogs everywhere, as far as I know. Certainly one could have a blog with tens or even hundreds of contributors, I suppose although would be rather unwieldy. Contributors can comment without moderation, but are subject to having their comments deleted by moderator.
Those who are allowed to comment on blogs - varies widely. Used to be that anyone could comment on virtually any blog with very little interference. Due mostly because of spam comments, comment moderation has become extremely common. I would guess there are very few active blogs that allow unfettered commenting. At the lowest threshold, there is a requirement that commenters "interpret" numbers/letters that are somewhat distorted which would weed out computer-generated spam. That is actually my least favorite - I don't think it is very effective and I, personally, sometimes have difficulty figuring out the correct numbers/letters. Second level, is requiring that all commenters be registered and log-in. The register may be specific to the blog or, like Blogger, be part of a much larger "group". To comment in Blogger, you have to have some sort of Google account - easy enough to work around if one really wanted to be anonymous, but generally not worth the trouble for most people.
The next level up of comment moderation is requiring moderator approval. On this blog, there aren't many comments NOT from contributors, so there is very little comment moderation to do. More "public" blogs - particularly ones that are controversial, political, social issues, etc. would get a lot of comments - many good and on-point, but quite a few that don't fit.
Some blogs do not allow any comments at all.