This can be done from a table of managers, where you'd have one record for each manager...
Or you can use your table of employees, but replace the body layout part with a sub summary layout part "when sorted by" the manager field. Summary fields placed in this sub summary part may be able to report your average grade--if there is a grade field defined in this employees table. You may need to add a calcualtion feld if the grade is computed elsewhere, that copies this value into the employees table so that a summary field can average it.
Please note that such a layout will be blank unless the records are sorted by the "sorted by" field of your sub summary layout part.