openSheet_checkUser.gs

var app = SpreadsheetApp.openById("your-sheet-id");
var remindSheet = app.getSheets()[0];   // Get's the fisrt table in the sheet

function checkUser(userId) {
  var lastRow = remindSheet.getLastRow(); // the index of the last row
  var inDB = -1;

  if (lastRow > 0) {
    var ids = remindSheet.getSheetValues(1,1,lastRow,1);
    
    /* 
    In our sheet, we will store all userIds in the first column with no header.
    getSheetValues(startRow, startColumn, numRows, numColumns);
    return (ids): [['userid1'], ['userid2'], ['userid3']]
    */
        
    for(var i = 0; i < lastRow; i++){
      if(userId == ids[i][0]) {
        inDB = i+1;   // programming list starts with `0`, but Sheet starts with row index `1` 
        break;
      }
    }
  }

  console.log("inDB = ", inDB);
  return inDB;
}