Groups of three is ideal. Three gives you one down, one to take care of the victim, and one to go for help. Two people can also pretty reliably carry out another even if the largest in the group is the one who can't walk.
Depends entirely on the people involved - time for a war story from way back when:
We received a call late in the evening from two hikers who had hiked out from the Galiuro Mountains, AZ, stating that their companion was hypothermic and had been left behind - as it turned out they gave his location accurately.
We responded immediately, reaching the trail head about 10 PM (quite a long drive from Tucson). Three of us immediately set out on snowshoes, traveling over about two feet of snow the last two miles to reach the location about 3 AM. Here we found the body of the victim - frozen stiff by the side of the trail where his companions had left him. No sign of any attempt at assistance (tent erected, fire built, etc). When he became distressed, his companions stopped, turned around, and departed the range by a longer, lower elevation route.
I later talked with the brother of the victim, giving him as much information as I could. i learned from him that the party had become wet the night before, sleeping out in the open without setting up the tents they were carrying. I was also told that the two surviving companions had recently taken a winter survival course after getting into difficulty on an earlier trip. I guess you could say their training was effective - after all, they did survive.
Be careful in your choice of companions. There is nothing inherently safe about a group vs solo.
Note also that this is also a situation where a PLB would have been of questionable value.