Go to this page: https://www.qualtrics.com/support/survey-platform/faqs/contacts/

You will find the answers for : 

• What is a contact list?
• How do I import a contact list?
• Why is my contact list not importing?
• How can I add extra information to my contact list (gender, city, department, etc.)?
• How do I update a contact list?
• Can I add information to a contact list after I have sent out my survey?
• If I accidentally added the same person in my contact list twice, will they go two emails? 
• If someone unsubscribes from one of my contact lists, will they automatically be unsubscribed from all my other contact lists?