As I’m sure you’re finding, it will always be suboptimal to try to loop in people who are not actually members of your site, if not impossible.
If you can, you want to invite these folks to join your discourse and set up default settings for them to watch the categories you expect them to participate in by email.
You can set up groups with default category notification settings to target these people, and add those people to the group as you invite them.
You can also write a topic to send them to the first time they log in that explains how they can participate by email. Then they never have to log in again.