We are using the Byddpress Follow plugin (https://wordpress.org/plugins/buddypress-followers/) to allow users to follow one another. We need to allow users to be able to also follow specific topics/categories, and have those posts be added to their feed. A perfect example of this is elitedaily.com: http://elitedaily.com/topic/presidential-election-2016/ They also use budyypress follow plugin. Please sign up with facebook on elite daily to see desired functionality. We have more Buddpress customization work immediately after if this goes well.