Bivying, or bivouacking to be correct, is the art of sleeping out without a tent or other shelter and is far older than mountaineering itself.
Anyone walking or climbing in conditions where there is a risk of getting hypothermia if benighted (and that includes winter climbing and walking in most of the UK) should, at the very least, carry a basic survival bag. And as a rule we would advise that each person should carry their own survival bag, rather than rely on one group shelter, as if anyone gets separated from the rest of the party, or has to go for help, they will still have their own emergency shelter with them.
For alpinists, a lightweight long-lasting breathable bivi bag is essential if they are considering tackling multiday routes, and could be a life-saver in severe weather.
Bivi bags with poles (or Bivi Tents) are not much used by climbers due to their extra weight and bulk, but appeal to the lightweight walking enthusiast who does not want to be burdened with a tent.
Group shelters are mainly (as their name implies) used to provide temporary shelter from the wind and cold for groups under instruction - but carrying a small version is worth considering by any winter mountaineer.