[Question] Subscribe module/snippet?

Greetings people,

Am finding a subscribe module or snippet that allows the registered user to subscribe to document ids, other user or TVs in the website. Also, able to view and show the updated subscribed list.

Anyone know any module or snippet out there that is capable of doing this?

I done a search and found nothing. Hopefully I do not need to code it myself; my programming skill is not good frankly.

Juz