Can I have a .csv file that is just a list of e-mail addresses (whicih happen to match the UserPrincipalName for users in our environment) and add those users a Group? 

I've tried "logonName" as CSV header and that doesn't work either.

I don't want our admin to have to manage a list of samaccountnames or Distinguished Name entries to enable automation of a distribution list.

It would be easier if they could just have a .csv file with userPrincipalheader and have the AD Group Automation add users from that csv to the specific group.