How could someone who has just enrolled in a phd or started a research could find online discussion groups, especially if the topic is relatively less popular.(e.g How to find research or discussion group working in privacy aware learning specially differential privacy).
If you are in a small niche area of a field then such groups aren't likely to be very visible. It might even be that none exist.
But, if there are productive people in that field who are still working actively, you can contact a few of them and ask what might be available. Recent papers can be your guide. It might even be that retired folks know of active researchers, perhaps among their own students, who can help.
And, consider starting such a group yourself if you can get a few interested people to agree. It might pay dividends in the future.