Sorry for resurrecting this topic but I’ve found myself in a similar situation.
I need to invite a lot of people to a category and I would like to have them notified by email by default (watching).
Maybe having a link I can send to people like /c/catname/subscribe so they click to get subscribed would be a good first step.
Edit: Related:
https://meta.discourse.org/t/category-based-watching-field-to-mass-add-remove-users/25617