Google Apps Script for a Slack Bot backed by a Google Sheet. https://blog.tuckerosman.com/2018/05/make-slackbot-from-google-spreadsheet.html

Code.gs

function checkingLoop() {
  //This is what checks every so often and sends the reminder message.
  const range = getNearestDateTimeRange(moment());
  if(!range) return;
  const leavingCell = range.getCell(1,2);
  const returnCell = range.getCell(1,3);
  const leaving = leavingCell.getDisplayValue();
  const returning = returnCell.getDisplayValue();

  if(leaving.length) {
    const leavingCars = leaving.split(";");
    leavingCars.forEach(function(element) {
      const leavingPeople = element.split(",");
      if(leavingPeople.length<2) return;
      var leavingMessage = "Hey "+normalizeName(leavingPeople[0])+", you're leaving";
      if(leavingPeople.length>1) {
        leavingMessage += " with "+listPeople(leavingPeople.slice(1));
      }
      sendMessage({"text":leavingMessage, "link_names": 1});
    });
  }
  
  if(returning.length) {
    const returningCars = returning.split(";");
    returningCars.forEach(function(element) {
      const returningPeople = element.split(",");
      if(returningPeople.length<2) return;
      var returningMessage = "Hey "+normalizeName(returningPeople[0])+", you're returning";
      if(returningPeople.length>1) {
        returningMessage += " with "+listPeople(returningPeople.slice(1));
      }
      sendMessage({"text":returningMessage, "link_names": 1});
    });
  }
}

function normalizeName(name) { //Maps to username and tags
  if(!name) return "";
  switch(name.toLowerCase()) {
    //Add case statements here for names that are different from usernames. Returns an @ reference for slack to resolve.
    default:
      return "@"+name.toLowerCase().trim();
      break;
  }
}

function listPeople(people) {
  if(people.length == 0) return "";
  else if(people.length == 1) return normalizeName(people[0]);
  else if(people.length == 2) return normalizeName(people[0]) + " and " + normalizeName(people[1]);
  else {
    var out = "";
    for(var i = 0; i<people.length-1; i++) out += normalizeName(people[i])+", ";
    out += "and "+normalizeName(people[people.length-1]);
    return out;
  }
}

function getNearestDateTimeRange(time) {
  const now = time || moment();
  //Determine the column
  if(now.day()<1 || now.day()>5) return undefined;
  var col = 3*(now.day()-1)+1;
  
  //Now let's figure out the row. We don't want to do anything too early, so let's keep it to 7 minutes or less before the designated time.
  if(now.hour()<7 || now.hour()>17) return undefined;
  //Now let's find the next 15 minute increment
  var next = roundToNextInterval(now); //Returns discrete moment
  var duration = next.diff(now, "minutes");
  if ((duration >= 60 && (duration - 60) > 5) || (duration < 60 && duration > 5)) return undefined;
  var row = (now.hour()-7)*4+3; //Get the base hour
  if(next.minutes()==0) row += 4;
  else row += next.minutes()/15;
  //Data source so it's statically linked for running in the background
  return SpreadsheetApp.openById("1sG4n2VwfDkcirrU3B8NnZ64T2T4irHcgwK92UeZZLLQ").getSheets()[0].getRange(row, col, 1, 3);
}

function roundToNextInterval(x) {
  const next = x.clone();
  const intervals = Math.floor(next.minutes() / 15);
  if(next.minutes() % 15 != 0)
    intervals++;
  if(intervals == 4) {
    next.add('hours', 1);
    intervals = 0;
  }
  next.minutes(intervals * 15);
  //next.seconds(0);
  return next;
}

function sendMessage(content) {
  const options = {
    "method" : "POST",
    "contentType" : "application/json",
    "payload" : JSON.stringify(content)
  };
  const url = "https://hooks.slack.com/services/..."; //Slack WebHook here.
  const response = UrlFetchApp.fetch(url, options);
  return response;
}

function test() {
  sendMessage({"text": "This is a test text message."});
}