function getLedger() {
  /*
  Return Google Sheets storing balance of users.
  */
  return SpreadsheetApp.openById(PropertiesService.getScriptProperties().getProperty("SHEETID")).getSheets()[0]
}

function getUserIndex(users) {
  /*
  Return associative array of {"user": "index in Google Sheets"}.
  If not found, the value becomes NaN.
  */
  const sheet = getLedger()
  const memberList = sheet.getRange(2, 1, sheet.getLastRow() - 1, 1).getValues()

  // Find user from "users" and push them to "userIndex" associative array
  const userIndex = {}
  for (var u in users) {
    userIndex[users[u]] = NaN // Set default value
    for (var m in memberList) {
      if (users[u] === memberList[m][0]) { // User found in the member list
        userIndex[users[u]] = parseInt(m) + 2 // +2 because the sheet has the header row and Google Sheet is 1-origin.
      }
    }
  }
  return userIndex
}

const userIndex = getUserIndex(recipients.concat([sender]))

const notFoundUsers = [] // If a recipient does not have acccount yet, notify later.
const ledger = getLedger()
for (var i in recipients) { // Add coins to each recipient
    const r = recipients[i]
    if (isNaN(userIndex[r])) { // Not member yet.
        notFoundUsers.push(r)
    } else {
        const v = ledger.getRange(userIndex[r], 2).getValue()
        ledger.getRange(userIndex[r], 2).setValue(v + amount)
    }
}

// Subtract total amount of sent coins
const current = ledger.getRange(userIndex[sender], 2).getValue()
const numPayedUsers = recipients.length - notFoundUsers.length
const payment = amount * numPayedUsers
ledger.getRange(userIndex[sender], 2).setValue(current - payment) // Update balance