how to manually subscribe user to comments

Hello,

I bought the Media Uploader addon for the plugin. Hope the premium support will fix my issue.

My website is made to organize events for my scubadiving club.
So my post are events, and I have setup a registration system to these events.
When a user is registered, he receives an email (this feature is made with php)

In addition, I wish to subscribe the user to the post comment.
The goal is that he is kept up to date with all comments of the post.

So my question is ; is there a function that does ;
Enable_post_subscription(user_ID, post_ID)
and the reverse (if the user unregister the event).
Disable_post_subscription(user_ID, post_ID)

Cheers
Alexandra