I agree we should be supporting both.
If the first item in the list has an “@”, then import by email, otherwise, import by username.
Also, we could be a little more lenient when it comes to the format of that list.
We could just require the emails/usernames to be on their own line and then do a split("\n"), remove any empty lines, remove any trailing “,” and remove any pair of single/double quotes (eg. gsub(/^['"]|['"]$/, "")).