In order for your participants to receive an automatic registration confirmation message as soon as they register, you have to create it before sending your invitation campaign. You can create a mail or a SMS campaign in  the tab Messages > registration confirmation messages. 

Personalize the email by using the "Nominative values" for example *|FIRSTNAME|*, *|NAME|* ...

In order to permit participant alteration in their registration, you must insert the Url of the registration form that will give access to the pre-filled form upon opening of the email.

Once your campaign is ready do not forget to change its statuts from "Paused" to "In Progress" in order for it to be sent automatically.