To collaborate on annotations one has to be sent a special link from the video uploader. There's no way to automatically display that link to viewers, unfortunately. I have my flickr page set up so that any user can add notes or tags but with YouTube I guess I'll have to post a link in the descriptions.
What are collaborative annotations good for? I don't know, you tell me. Follow this link to compose annotations for the video below. This will either turn out very funny or very mean. Either way it should be interesting. Or not...
(Yes, fanboy, I am aware that Nico Nico Douga had video annotations since forever.)
YouTube Blog - Introducing Collaborative Annotations
Nico Nico Douga
Cat solves printer-problem(with subtitle)