Most of them are getting pushed there from automated platforms bombarding them with things they can't stand. The automated outrage machines are what radicalize people, not the private message groups.
By the time you have private message groups propagating the same messaging the damage has already been done. To get people out, you need to cut off the source of the outrage and soothe whatever fear is being ramped up.
Censoring the outrage and dismissing/patronizing people from a position of authority doesn't work, it just acts as an accelerant.
I think the crazy groups will sputter out if there's enough space for venting, free association, and ability for the average person to turn off things they don't want to see. I think that last point is the key. Millions of people are bombarded with things they really don't like in systems they have no way to really manage themselves. The legitimate complaints will filter up and out of the crazy circles if people are allowed to form their own groups and don't feel forced to pack themselves together around the loudest people who can given them a voice on the big automated feeds they don't really understand.
People have a much better chance at effectively communicating with their neighbor or family member or old friend with a different perspective than a stranger online with a different perspective, and they are much more likely to engage when in a frame of mind that will actually accept differences of opinion if they are managing their own connections and exposure themselves.