How to get contributing authors to sign up

I am the editor of a magazine that is looking to feature contributions from our readers as blog posts.

What I understand is this: We need to create contributing author accounts for people to upload their own content.

The problem is this: We now have 15 contributing authors--all of whom we have signed up "manually". We want to increase our number of contributing authors to 100-125. Ideally, they will visit a page on our site, be able to create their own contributing author account, and begin uploading their own content straightaway.

Is there a plugin for this?