This API uses an oAuth2 approach. With the endpoint /sites/$site/follows/new, it looks like you’d have to prompt the user to authenticate themselves on WordPress.com, get back a code, which you then have to exchange on an endpoint for access token. This is all explained here. The information stored in the Access Token you receive after the oAuth2 authentication would hold the user’s information, and would be used to add them as a follower of your blog when you make the POST request.
Unfortunately, in as far as I can tell, there is no way to add a follower by email through the API directly.
I know it may not be the ideal solution, but it looks like your best bet is to create a follow button here: https://en.support.wordpress.com/follow-button/
Of course, you could go through the whole oAuth2 approach, but ultimately it’d probably be a lot easier to just generate the button. Hope this helps.