How to find online discussion groups on research topics which are relatively less popular
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.