This recipe is an add-on for the Events recipe, adding the ability for site visitors to register for events, and for site admins to manage the registrations. It does so by providing an initial configuration of the Entity Registration module, using a simple "Signup" registration method. This can be customized by adding fields for information needed during registration, such as t-shirt size or food allergies. Once applied, this recipe will set all new events to use registration, in other words "opt out" if a particular event doesn't need it. You can change this to "opt in" if you prefer by going to /admin/structure/types/manage/event/fields/node.event.field_signup and setting the default Registration type to "--Disable registrations--".
Link