Storm shelters, also known as mountain shelters, bothy bags or group shelters, but all names refer to the same piece of kit.
I first came across one of these while on my BEL course (Basic Expedition Leader) on the Pentland Hills, many years ago. It was a wet and windy day, and the Instructor got us all undercover using a storm shelter while we had lunch. Within 15 minutes or so, I was so warm underneath this shelter; I had to take my jacket off. Since that day, whenever I go out with a group, I ensure that I have my group shelter packed!
Storm shelters come in various sizes, from one person, up to eight or ten. If leading a group, you will need to ensure that you have a correct ratio of shelters to people. If not on a cold, windy raining day, you may have a few disgruntled participants :-)