Importing users via username/email etc. is very handy, but users must be created/imported and exist on the system prior to launching an import process for a group or course membership.
In the old user management, non-existing users have been marked as "don't exist and therefore will not be added". In the new user management in groups and courses this behavior changed, not existing users are just not imported (correct), however the user is not notified by any mean about the fact that some users have to be created before importing (wrong).
When adding just a couple of users this is no big deal, however when you have 163 users to import and two of them do not yet exist on the system it is almost impossible to 1) be aware of this fact and 2) find out which two users are the ones that do not exist.
The missing should be displayed below or above the table where the user confirms the import of existing users. In the feedback after importing this information should be repeated as well.