function writeAttendeeAssignments_(ss, attendees) {
  let sheet = findOrCreateSheetByName_(ss, 'Activities by person');
  sheet.clear();
  sheet.appendRow(['Email address', 'Activities']);
  sheet.getRange('B1:1').merge();
  let rows = attendees.map(function(attendee) {
    // Prefill row to ensure consistent length otherwise
    // can't bulk update the sheet with range.setValues()
    let row = fillArray_([], ACTIVITIES_PER_PERSON + 1, '');
    row[0] = attendee.email;
    attendee.assigned.forEach(function(activity, index) {
      row[index + 1] = activity.description;
    });
    return row;
  });
  bulkAppendRows_(sheet, rows);
  sheet.setFrozenRows(1);
  sheet.getRange('1:1').setFontWeight('bold');
  sheet.autoResizeColumns(1, sheet.getLastColumn());
}
Populates a sheet with the assigned activities for each attendee.

@param {Spreadsheet} ss - Spreadsheet to write to.
@param {object[]} attendees - Array of attendees with their activity assignments