In the case of WhatsApp, Times of India reports that group administrators are directed to remove members from a group chat if they're breaking the law. If that doesn't happen, the admin shoulders the responsibility and is considered guilty by proxy. More than that, group chats are supposed to only include people that the admin knows personally.
With this most recent arrest (there was at least one prior; immediately after the law was passed), admin Krishna Sannathamma Naik was detained after another group member, Anand Manjunath Naik, reported Krishna and two other members of the group to the police. Ganesh Naik has been arrested and is out on bail, and Balakrishna Naik is on the lam. It isn't clear if the members are related, but man, if they are, ratting out your family over something like this is kind of cold.
India's apparent justification for the law is that since WhatsApp is incredibly popular in the country -- it has over 200 million users, according to News18 -- the chances of fake news going viral on the platform are high. While that's noble, this could be seen as censorship and the state impeding free speech.