Add the user's email to a SendGrid contact list on sign up.
### How this function works
This function will allow you to add a new contact with the user email to a SendGrid contact list when a new user signs up.
* You must have previous knowledge of **SendGrid**.
### Function details
This function listens when a new user signs up and checks if an email is available according to the provider used to register. If an email is available, it will add a new contact with that email to the specified SendGrid contact list.
To install this function you need to enter the values of the following variables:
* **SENDGRID_API_KEY**: The API key to authenticate access to SendGrid services.
* **SENDGRID_LIST_ID**: The ID of the list to add the contact.
* [SendGrid contact API](https://sendgrid.api-docs.io/v3.0/contacts/add-or-update-a-contact)