The option to upload a CSV file to your contacts or invitation is currently unavailable. However, this is an option that we'll be adding in the near future, so please check back with us. In the interim, you can simply copy and paste your guest's email address onto the guest list of your invitation. You can copy and paste up to 100 email addresses at a time. After clicking "Add to Guest List," you can paste another set of 100 email addresses and so on.
Once you send your invitation, these contacts will automatically be added into your Contact list. You'll then be able to easily create a future invitation with your saved contacts.