Swayamsevak is a Sanskrit term for volunteer.
It is also a term used for volunteers of Rashtriya Swayamsevak Sangh. Members of RSS are called swayamsevak. There is no formal membership to become a swayamsevak. Swayamsevak has to be a patriot and they encourage other citizens to be the same. It is also the world's largest voluntary organisation according to BBC.
References
RSS swayamsevak Wikipedia(Text) CC BY-SA