There are no registrations or checkpoint because most of the people attending a wedding might not have even received a formal invitation to attend the function. Usually, the number of people or guests attending an Indian wedding is more than 500, and it is a regular affair to attend marriages in India.
Since there aren't any registration procedures or checkpoints, it's likely that many people attending an Indian wedding haven't even been formally invited.  Indian weddings are known for their large guest lists, often exceeding 500 attendees, and it's customary to attend such celebrations in India.