Adding an extra category list

Evening,

A few months ago I started a group blog, and instead of having cateogories, I thought I would replace that function with a list of “authors” instead. So the category function lists posts by author instead of by topic. Now that I received several hundred entries, I’d like a way for me to group these by category and not just by author. However, I can’t find a way to do this without having to edit every single post and having to create multiple user accounts.

Other than technorati tags, is there any way for me to do this? Tags is a good idea, but I need these categories to appear somewhere on the site so that visitors will read the topics of interest.

I have over 20 writers and several hundred posts, so I can’t consider re-placing the category list with how it’s really supposed to function. It’s too time-consuming.

Any help would be greatly appreciated, thanks.