I just saved one sim from Grim after she collapsed from freezing. She was on the street in front of my sim's house and my sim convinced her to group up. It would take too long to walk back to my sim...
I'm having this problem too. I had a packed graveyard full of NPC's to the point where I had to put my family in a new save. I have so many mods and custom content, but the main problem is the sims themselves are going jogging in the winter. Hopefully a mod can come out to end winter jogging. It's unbearable to think that as my sims expand the family, that those unplayed family members will freeze to death. :(