SUM.gs

/**
 * Counts a guest's name that is not highlighted nor has strikethrough
 *
 * @return The value of all the guests.
 * @customfunction
 */
function COUNTGUESTS(){
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = SpreadsheetApp.getActiveSheet();
  var formula = SpreadsheetApp.getActiveRange().getFormula();
  var args = formula.match(/=\w+\((.*)\)/i)[1].split("!");
  try {
    if (args.length == 1) {
      var range = sheet.getRange(args[0]);
    }
    else {
      sheet = ss.getSheetByName(args[0].replace(/'/g, ''));
      range = sheet.getRange(args[1]);
    }
  }
  catch(e) {
    throw new Error(args.join("!") + " is not a valid range");
  }
  var weights = range.getFontLines();
  var values = range.getValues();
  var colors = range.getBackgrounds()
  var guests = 0;
  
  for (var i = 0; i < values.length; i++) {
    for (var j = 0; j < values[0].length; j++) {
      if (weights[i][j] != "line-through" && colors[i][j] == "#ffffff" && values[i][j] != "") {
        guests += 1;
      }
    }
  }
  return guests;
}