I've grouped people via a sort by Last Name.
Won't that produce incorrect results if you get two people with the same last name?
Please describe the structure of your data in this table.
There are two possible approaches:
One options is to use a method called "sum the reciprocal". It's not a simple thing to implement, careful attentiong to detail is required, but it does work: How to count the number of unique occurences in field.
For uses with FileMaker 12, there's also a way to get these counts using ExecuteSQL(): A new way to count unique values in FileMaker 12