Assuming your users can see each other’s profiles:
Make a new boolean column on the profile called “Follow this user” or something similar. Make it user specific, and use a switch for the user to select “Follow”
To be able to see the list of people you follow, all you need is an inline list pointed at the profiles or user pages, and filter it like this:
As far as seeing who is following you, I haven’t worked that out, but I don’t need to. I just wanted to be able to follow certain people, not see who was following me.