Multisite - allow users to sign up as a user (subscriber) to individual blog

I'm stumped on something that I suspect is super simple. I want my users to be able to sign up as a subscriber (by default) user for other user's blogs. All I can figure out how to do is register as a user of the network, but these users are already network users.

Basically I'm trying to link users together through subscription, so that I can present a user with a page of recent posts from only blogs that they are a subscriber for. I'll also want to use this for other user roles, but that's the next step.

I would greatly appreciate your help, even though I don't think this relates to one of your plugins.

Thanks so much