Looking for enroll to post (i.e. task)

Hi,

I am making a kind of project management system and I want to be able to let users enroll for a task. So for example a task is to write an article and therefor I need a writer and a photographer. If a user wants to be a writer he/she can enroll on the task as a writer.
But I cannot find a plugin or any other solution for it. It would be great if it is a field or something similar which I can add to the post area.
Anybody with a suggestion?
Thanks in advance!