All foreign nationals entering India are required to possess a valid international travel document in the form of a national passport with a valid visa obtained from an Indian Mission or Post abroad.
An invitation letter can be requested from the Program Co-Chairs of the conference and upon its receipt, the attendees have to fill up VISA form and apply at an Indian Embassy/Consulate.
More details about VISA and its application procedure can be found at Indian Visa Website.
FAQs on Conference VISA can be found here.