In what group of Kayan people is this tradition (neck rings)?

The Kayan are a sub-group of Red Karen (Karenni people), Tibeto-Burman ethnic minority of Myanmar (Burma). The Kayan consists of the following groups: Kayan Lahwi (also called Padaung, Kayan Ka Khaung (Gekho), Kayan Lahta, Kayan Ka Ngan. Kayan Gebar, Kayan Kakhi and, sometimes, Bwe people (Kayaw).

Padaung (Yan Pa Doung) is a Shan term for the Kayan Lahwi (the group in which women wear the brass neck rings).

Women of the Kayan tribes identify themselves by their forms of dress. Women of the Kayan Lahwi tribe are well known for wearing neck rings, brass coils that are placed around the neck, appearing to lengthen it.

Girls first start to wear rings when they are around 5 years old. Over the years, the coil is replaced by a longer one and more turns are added. The weight of the brass pushes the collar bone down and compresses the rib cage. Many ideas regarding why the coils are worn have been suggested. Anthropologists have hypothesized that the rings protected women from becoming slaves, making them less attractive to other tribes. It has also been theorised that the coils originate from the desire to look more attractive by exaggerating sexual dimorphism, as women have more slender necks than men. It has also been suggested that the coils give the women a resemblance to a dragon, an important figure in Kayan folklore.The coils might be meant to protect from tiger bites, perhaps literally, but probably symbolically.