I’m intrigued that you misunderstood how the invite to topic feature works - we will be reworking this feature to bring it in line with the newly improved invite system. Hopefully in the process we can make it less confusing.
This feature request enters staged user territory. Staged users are not really users because they can only participate by email, and staged user functionality is not well fleshed out except in the PM system. I think you are asking to push the limits of what staged users are intended to do which is interesting but not likely to happen anytime soon if ever.
For most communities it is nonsensical to have people participating in discussions who are not users. There is a slippery slope here as well… what user privileges should we give staged users next? Staged users cannot be @ mentioned, they do not watch topics or categories etc, they don’t have access to attachments in private categories. They can’t log in to edit their own preferences, including email preferences. They can’t get weekly email summaries. They don’t show up in user lists. When do staged users just become users who never log in? We want the forum experience to be awesome for everyone, so really the goal should be to lower the barriers to getting people to log in, rather than letting them hide in their email.
Can you elaborate a bit on your use case, and what you’ve already tried with existing Discourse functionality and where you are running into trouble? Can you describe these people you are trying to loop in and why they are not willing or able to sign up and participate as users?
A use case I can imagine for me personally is to invite family members and friends who are reluctant to join the private discourse site I am using to manage family projects like photo albums and such. I know they will be excited to hear about isolated topics, add replies and talk with me and maybe a few other people in those topics. However, they will not ever want to join the site and be burdened by logging in to reply and also seeing all the other topics they are not interested in. But this is a fairly particular and likely rare use of Discourse, and I am able to deal with it by just emailing these people separately to talk to them about my projects.
Another use case might be in Discourse for Teams, where we are already experimenting with guest user functionality. At the moment, the way guests interact with Teams sites is by being invited specifically to join guest categories. Guests are counted separately from the user limits (the same number of guests can be invited as team members). Topics can then be created in the guest category that are isolated from the rest of the site, allowing the team to engage with guests on specific topics, e.g. on client projects. So far we have not seen many Teams sites using the guest functionality, but they might start using it if they saw an easy interface to use to invite guests to topics and let them join it via their email without having to log in.
I guess we could achieve what you describe by following the example of how staff can PM email addresses, creating staged users who can then reply and get emails whenever their PMs are replied to - and if they decide to create an account later, they gain full access to their activity as former staged users. A button could be provided to send a post to an email address, maybe with a personal note at the top. A staged user would be created and the full post would be sent to them by email. If they reply, they’d then be watching the topic so they get future replies (this came up in another topic this week). If they are overwhelmed they can use the unsubscribe link in the footer of the email.