Changing level required to post

I was looking at a hack or plugin a while back that allowed me to make the equivalent of Livejournal friends only posts by restricting some posts to be viewable only to people who have registered an account on my blog. I never ended up using it, because it seemed that registering an account and giving them a level of 1 meant that they could post to my blog, which is something I don’t want them to be able to do. Level 0 didn’t work, because then I couldn’t control who was able to create accounts to view the posts in question.
For that reason, I never did get the hack installed that added this functionality, but it’s something I would still like to add. So, is there any known way of altering the level someone needs to have to be able to post to the blog? If there is no default WP functionality to do it, is there a hack or plugin out there that might do the job?
Any help is appreciated.
Thanks,
Ray