The Drupal 8+ eco system doesn't have a great system for managing event registrations and attendance, that allows users to pay for event registrations through the Drupal Site. Our major community events, including DrupalCon and Drupal Camps leverage external systems.
The new Event Registration and Event Ticketing modules provide a framework to build a robust event management system within your site. Together they provide free and paid event registrations, tracking of attendees, and check-in and management of activities during the event. Simply enabling Drupal Commerce's checkout process allows for easy setup of payments using the supported payment gateways or custom workflows.
Additionally the event registration module is designed to handle not only individual attendees, but also speakers, sponsors, and supporting persons and organizations. Through the extensibility of Drupal Commerce and entities, these modules can provide a solution for almost any event situation.
Rich Gerdes of Unleashed Technologies