A large group of people populate a dimly lit room.
A group of people are partying in the large room.
Just because it is a large group of people, does not mean they are in a large room. Just because the group is in a dimly lit room, does not meant that they are partying.
Just because a large group of people are in a dimly lit room doesn't mean they are partying.
A group of people can get together without partying.