Group dancers are also called chorus or backup dancers. Clearly, without them most of the dance sequences would not be too interesting. The reason is that solo performances by the lead characters are rare and usually occur in formal, classical dance settings. For an average movie-goer, these are not very appetizing. Most of the time, though, the main dancer or dancers have to be embedded in a matrix of backup dancers.

In the talkies, when the dances moved away from the temple or the court to other locations, a need arose for more dancers. These dancers were, unfortunately, very low in the movie industry hierarchy, just above the extras. The extras are the ones sitting in the cafeterias or in parties. You can actually recognize many of them who are common fixtures sometimes for years. Little bit above them are the unheralded group dancers. They are usually hired by dance directors, sometimes called the dance masters or simply masters. They work in groups, get to know each other and typically do not need much rehearsal. They work is reasonably hard, very enjoyable for those who love dancing and pays well, roughly about 5-10 times a secretarial or low-level desk clerk job.
Sadly not much has been written about them. One can forget about biographies, for the most part, even their names are unknown to an average film-goer.
This book is a small effort to name them and have their filmographies written down. Much work still needs to be done.