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
Depending on what flavor of "DemSoc" they are, from SuccDems to actually socialist Kautskyites, you might need to be cautious. Better to play it safe and maintain a neutral tone, and not instantly resort to revolutionary rhetoric. When accused of "simping" for China or some [bad AES country], I just resort to "I can't do anything about there whereas I live here".
Hell, on foreign policy, most SocDems at least oppose more egregious instances of US imperialism like the Cuban Embargo, so you may be able to find a wedge with some probing.