visitors creating user names

Aug 19, 2009 at 8:53 PM

i would like to be able to have visitors create a username and password but still have only the ability to comment on my posts.  i would like this option because other parts of my site include using usernames such as my chatroom, and i would like to keep it all the same without having the visitors retype the username everytime they go to a different page. how could i go about doing this?