function writeActivityRosters_(ss, activities) {
  let sheet = findOrCreateSheetByName_(ss, 'Activity rosters');
  sheet.clear();
  var rows = [];
  var rows = activities.map(function(activity) {
    let roster = activity.roster.map(function(attendee) {
      return attendee.email;
    });
    return [activity.description].concat(roster);
  });
  // Transpose the data so each activity is a column
  rows = transpose_(rows, '');
  bulkAppendRows_(sheet, rows);
  sheet.setFrozenRows(1);
  sheet.getRange('1:1').setFontWeight('bold');
  sheet.autoResizeColumns(1, sheet.getLastColumn());
}
Populates a sheet with the rosters for each activity.

@param {Spreadsheet} ss - Spreadsheet to write to.
@param {object[]} activities - Array of activities with their rosters