A valid CSV file import fails (exception thrown) if it contains a column that is unmapped.

username,first name,last name,email,groups
delme,del,me,del.me@asdf.com,agroup

If this would work one could use the same file for the user and group mapping just buy choosing the appropriate columns from each.

Another annoyance: I can't find a way to import users into more than one group at a time.