jq '
  [ 
    .messages[] | 
    {from, date} | 
    select(.from != null) 
  ] | 
  group_by(.from) | 
  map({ 
    from: .[0].from, 
    count: length, 
    firstDate: (map(.date) | sort | .[0])
  })
' export.json >authors.json
python3 output.py authors.json