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Once you've got the file open in Notepad, the first problem with
Groups.io's export is that it uses the unix-style line termination: just
a LineFeed (U+000A). Notepad doesn't recognize those as new lines, so
the rows of the export get mashed together. This should be taken as a
bug report (or feature request) - the export should use (or offer) ASCII
standard line termination - aka CRLF (U+000D, U+OO0A) for broadest
interoperability.

Fortunately Groups.io's export has the email address as the first
column, and the display name as the second. Which (had it not been for
the line termination issue) would have made it relatively easy for the
Notepad user to go down from line to line, deleting the unneeded
information from the end of each line.