This is precisely my user profile as well. They want to engage in just one topic, for example, but don’t need to or want to see everything else. This is actually quite common for us because we’re a grassroots organizing organization. We’re always interacting with people who are at the periphery of our network whose involvement is necessary in some specific instances but who would be overwhelmed by any more information.
Yes, this is exactly what I think would work and what I had assumed inviting someone to a topic would do when done via email because it mirrors how personal messaging to an email works.