The Mailing List holds the contacts for one specific event. If you create a new event, your mailing list will be empty, but (if you had a prior event) you will have all the contacts in your Address Book
Your Address Book holds all the contacts that you have ever added or imported to Greenvelope. Even if you delete a contact from your Mailing List, it will remain in the Address Book.
*NOTE* You cannot add guests directly to the address book. Instead, please visit the Send step within an event to add directly to the mailing list.