So, Facebook just made my partner the sole head of a group with more than a few thousand DemSocs in it
What steps should we take to achieve some actual radicalization among these potential recruits?
I mean, in addition to suggesting they join HexBear of course
None of this is as radical as it could be, but radicalization doesn't start at the end goal. Try to move a lot of people a little bit (and count on them to keep going down that path) rather than finding an advanced minority and catering to them. If that advanced minority is upset, reach out and explain your intentions.