My group is small enough that I can add only people I recognize. But for
a larger group, I wonder if you could get Facebook to do a lot of the
gatekeeping for you -- there is some sort of option where people applying
to join the group have to answer questions, and you could ask how long have
you been dancing contra/ECD? and what group(s) do you usually dance with?
I suspect that bots and trolls would usually not be able to answer such
questions convincingly.
regards,
ajr, dancing in and near Toronto, Canada