I found an answer to my question searching the forum but post is 5 years old. I will ask again:
I am using The Events Calendar Pro with an RSVP add-on. I would like all Members that have RSVPed to an event receive comment/message notifications without waiting for member to subscribe to comments. Is it possible to add member email to subscription list? (They have already opted in to receive event-related emails)
If not I do have a "switch to user" plugin that allow me to do it as the Member, but wondering if you have another solution.