How to have members

I posted an article and an accompanying assignment on each of my blogs but I'm a little confused as to how to have members rather than have the blog open to the public. I worked around this by giving instructions to students on how to comment on the blog rather than create a new post. I need to do more research to figure it out--I guess i should read the whole handout on blogs! It will be exciting to use this tool once I make it as user friendly as possible for the students.