Create a quarter entity with the right begin and enddate of that quarter. I would create a reference between this entity and the employee entity. Now create a microflow where you iterate over all the employee's. You can then use a XPath contstrained to retrieve the right quarter by checking that the day that the person joined should be bigger or equal with the start date of a quarter and smaller or equal to the enddate of that quarter. If you find the quarter then add this employee to the reference. This way you can easily select a quarter and see which employee's belong to that quarter. And do make sure that when creating a new employee you set this quarter in some after creation microflow.