Email Notification and Process Question

Our business requirement for our client is to notify users (via email) when certain files are uploaded. It seems like your plugin does this but I wanted to clarify how you control who gets notified when a file is uploaded and/or changed? More specifically, how are the contacts entered into the system and how are they managed? We would need to be able to have users opt-in via sharing their email address and then they'd be on the list to be notified. Thanks for any clarification you can provide.