This information should be coming from users who have registered on website. Because they are already registered we already have their e-mail, name etc
Looking at your domain model it looks like the person who wants to sign up for the event is not yet known in the system upon initial registration. If you have not yet stored any information, it is not going to be possible to fetch the details. If you created an account/user record after the first time a person registered to an activity, you can create an association between your saved person data and the activities. You can take a look at this section of the RAD course to see how you would go about doing somethin similar. If you want to know how to create an association to the actual account of the person that logged in, you could take a look here.