=SUMIF($A$6:$A$13,G6,$C$6:$C$13)

You just want to add all the numbers for a given manager... in my formula, I
have the manager names listed in column G...

For all the names in column A, find Manager1, and for all occurrences of
Manager1, add the numbers in column C (for simplicity, I just directly
substituted into column C the values 1 or 0).

Or in your example SUMIF(A1:A50,G1,D1:D50) where G has the manager names...

Worked for me...