Which group introduced and developed the caste system in India?

Caste System was not intentionally introduced by a specific group. It came into existence naturally similar to class system in modern world. Those who were teachers, scholars, priests were considered respectable by all and thus made into a higher caste, on the other side, the sweepers and cleaners, whose work was considered filthy and unworthy naturally formed a low caste group.

First answer: (by Abu Koroma) -->

the aryan invasion of indus and gandes led to development of the caste system