Add a 'follow' button functionality on Wordpress posts - repost

I have a user-based site at [login to view URL] When users log in, they can view posts and comment. I'd like to add a new functionality that allows them to 'follow' posts. Functionality: -Gif button that says follow under the post title on each post. -When clicked, button says 'following'. User's database entry is appended to include the post ID as a post they are 'following'. -If clicked again, button reverts to original state. User's database is edited to remove post ID from 'following' entries. I should be able to download the user database and see what posts are being followed by each user. You can login to check out the site with the following credentials: [login to view URL] user: guest pass: guest