I have an Excel spreadsheet with the following columns:

Employee Name; Employee Category; Supervisor Name; Supervisor Email

Obviously many employees may have the same supervisor. So the same
supervisor name shows up in multiple rows of my spreadsheet.

I want to do an email merge that sends ONE email to each supervisor, listing
all the employees under that supervisor, like this:

-----
Dear <Supervisor Name>,

Please complete evaluations for the following employees under your
supervision:

<Employee Name>: <Employee Category>
<Employee Name>: <Employee Category>
<Employee Name>: <Employee Category>
<Employee Name>: <Employee Category>
-----

How can I do this?