Subscription and Node action I am using Drupal 7. And i am quite new. 

I want to develop a system in website where an Event is declared by the Administrator. (Event is a custom event type - or i can use any existing one if needed).  The event has a field (variable number of) subscribers to it. 

If a user want to subscribe to event he/she can fill in a web form. Alternatively Subscription can be done by having a button (if such a thing is possible on node). 

The problem is, usually such triggers help send an email post this. However, i want that once such an event happens, i want to modify the target node and append the current user_reference in the list of subscribers in the node. 

So basically, when a user subscribes the page, the system modifies the node to append the name of such a user in the subscriber's list.

Is such a thing possible? If so, how?
Short answers - 

Drupal 6: Use Signup

Drupal 7: Use Entity Registrations

Both of these modules do pretty much exactly what you want on both counts.