Add followers' list at the end of discussion
You can do this using Flag module.


  Flag is a flexible flagging system that is completely customizable by
  the administrator. Using this module, the site administrator can
  provide any number of flags for nodes, comments, users, and any other
  type of entity. Some possibilities include bookmarks, marking
  important, friends, or flag as offensive. With extensive views
  integration, you can create custom lists of popular content or keep
  tabs on important content.


Creating Following &amp; Followers